Display device

ABSTRACT

An object is to reduce an occupied area of a protection circuit. Another object is to increase the reliability of a display device including the protection circuit. The protection circuit includes a first wiring over a substrate, an insulating film over the first wiring, and a second wiring over the insulating film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a protection circuit and a displaydevice having thereof.

2. Description of the Related Art

In recent years, technology that is used to form thin film transistorsusing semiconductor thin films (with thicknesses of from severalnanometers to several hundreds of nanometers, approximately) that areformed over substrates having an insulating surface has been attractingattention. Thin film transistors are applied to a wide range ofelectronic devices such as ICs or electro-optical devices, and promptdevelopment of thin film transistors that are to be used as switchingelements in display devices, in particular, is being pushed.

As a switching element in a display device, a thin film transistorincluding an amorphous semiconductor film, a thin film transistorincluding a polycrystalline semiconductor film in which the diameter ofthe crystal grain is 100 nm or more, or the like is used. As a switchingelement in a display device, further, a thin film transistor including amicrocrystalline semiconductor film is used (see Reference 1: JapanesePublished Patent Application No. H4-242724 and Reference 2: JapanesePublished Patent Application No. 2005-049832).

In a semiconductor element typified by a thin film transistor, it is oneof important objects in a manufacturing process of a semiconductordevice that how an electrification phenomenon (charging) which leads todeterioration in an element or a dielectric breakdown is suppressed.

Causes and circumstances of charging are extremely complicated anddiverse. A protection circuit using an impedance element such as a diode(a protection diode) or a resistor is used to prevent deterioration or adielectric breakdown due to charging. When a discharging path isprovided by using the protection circuit, charge accumulated in aninsulating film can be prevented from being discharged near asemiconductor element or at the intersection of wirings, so that aphenomenon where the semiconductor element deteriorates or is damaged bydischarge energy (ESD: electrostatic discharge) can be prevented(Reference 3: Japanese Published Patent Application No. H8-027597).

Further, when the protection circuit is provided, even when noise aswell as a signal and power supply voltage is input, a malfunction of acircuit of the next stage due to the noise can be prevented anddeterioration or damage of the semiconductor element due to the noisecan be prevented.

SUMMARY OF THE INVENTION

A thin film transistor having an amorphous semiconductor film in achannel formation region has lower field effect mobility than a thinfilm transistor having a microcrystalline semiconductor film or acrystalline semiconductor film in a channel formation region. That is,current drive capability is low. Therefore, when a protection circuit isformed using a thin film transistor formed using an amorphoussemiconductor film, a large-size thin film transistor is forced to beformed as countermeasure against an electrostatic breakdown, which leadsto hindrance to narrower frame parts of a display device, unfortunately.In addition, in an inverted staggered thin film transistor, a sourceelectrode and a drain electrode are overlapped with and electricallyconnected to an amorphous semiconductor film that is formed over a gateelectrode or a scan line, with a gate insulating film interposedtherebetween. Further, when a large-size thin film transistor is formed,there is a problem in that an area where a gate electrode or a scan lineelectrically connected to the gate electrode overlaps a source electrodeor a drain electrode, or a signal line electrically connected to thesource electrode or the drain electrode is increased, and electriccapacitance between the scan line and the signal line is increased,whereby power consumption is increased.

When a resistor is used for a protection circuit, there are problems inthat, in a similar manner to the case of a protection circuit where theresistor is short-circuited over a plane, it is necessary to have asufficient area of the protection circuit, the occupied area of theprotection circuit is large in an element substrate, and an area of adisplay portion is small.

In order to manufacture a display device through a small number of stepswith high yield, it is preferable that a switching element in a pixeland a protection circuit be formed at the same time.

In view of the foregoing problems, an object is to improve electriccharacteristics of a protection circuit and to reduce an occupied areaof the protection circuit in a display device. Another object is toincrease the reliability of a display device having a thin filmtransistor.

A protection circuit includes a first wiring formed over a substrate, asecond wiring intersecting the first wiring, and an insulating filmformed between the first wiring and the second wiring. One of the firstwiring and the second wiring projects into a side of the other of thefirst wiring and the second wiring at an intersection of the firstwiring and the second wiring. The insulating film formed between thefirst wiring and the second wiring is provided with one of a depressedportion and a separation portion at the intersection of the first wiringand the second wiring. In this region, one of the first wiring and thesecond wiring projects into the side of the other of the first wiringand the second wiring.

A plurality of first wirings extending in a first direction and beinglined up in a second direction intersecting the first direction over asubstrate, a second wiring extending in the second direction over thesubstrate, and an insulating film that insulates the first wiring andthe second wiring are formed. One of the first wiring and the secondwiring projects into a side of the other of the first wiring and thesecond wiring at an intersection of the first wiring and the secondwiring.

A protection circuit includes a plurality of first wirings and a firstcommon line each extending in a first direction and being lined up in asecond direction intersecting the first direction over a substrate, aplurality of second wirings and a second common line each extending inthe second direction and being lined up in the first direction over thesubstrate, and an insulating film formed between the first wiring andthe first common line, and the second wiring and the second common line.One of the first common line and the second wiring projects into a sideof the other of the first common line and the second wiring at anintersection of the first common line and the second wiring. One of thesecond common line and the first wiring projects into a side of theother of the second common line and the first wiring at an intersectionof the second common line and the first wiring.

A protection circuit includes a plurality of first wirings and a firstcommon line each extending in a first direction and being lined up in asecond direction intersecting the first direction over a substrate; afilm formed over at least the first wiring and the first common line; aninsulating film formed over the film; and a plurality of second wiringsand a second common line each extending in the second direction andbeing lined up in the first direction, formed over the insulating film.The film formed over at least the first wiring and the first common lineis provided with one of a depressed portion and a separation portion atan intersection of the first common line and the second wiring. The filmformed over at least the first wiring and the first common line isprovided with one of a depressed portion and a separation portion at anintersection of the second common line and the first wiring.

A protection circuit includes a plurality of first wirings and a firstcommon line each extending in a first direction and being lined up in asecond direction intersecting the first direction over a substrate; afilm formed over at least the first wiring and the first common line;and a plurality of second wirings and a second common line eachextending in the second direction and being lined up in the firstdirection, formed over the film. The film formed over at least the firstwiring and the first common line is provided with one of a depressedportion and a separation portion at an intersection of the first commonline and the second wiring. The film formed over at least the firstwiring and the first common line is provided with one of a depressedportion and a separation portion at an intersection of the second commonline and the first wiring. Note that as the film formed over at leastthe first wiring and the first common line, there are an insulatingfilm, a semiconductor film, and the like.

A display device includes a pixel portion and a peripheral portionprovided at a periphery of the pixel portion over a substrate. A thinfilm transistor and a pixel electrode connected to the thin filmtransistor are included in the pixel portion. A first wiring formed overthe substrate, a second wiring intersecting the first wiring, and aninsulating film formed between the first wiring and the second wiringare included in the peripheral portion of the pixel portion. One of thefirst wiring and the second wiring projects into a side of the other ofthe first wiring and the second wiring at an intersection of the firstwiring and the second wiring.

A display device includes a pixel portion and a peripheral portionprovided at a periphery of the pixel portion over a substrate. Aplurality of first wirings extending in a first direction and beinglined up in a second direction intersecting the first direction over thesubstrate, a second wiring extending in the second direction over thesubstrate, and an insulating film formed between the first wiring andthe second wiring are provided. A thin film transistor and a pixelelectrode connected to the thin film transistor are included in thepixel portion. One of the first wiring and the second wiring projectsinto a side of the other of the first wiring and the second wiring at anintersection of the first wiring and the second wiring in the peripheralportion.

A display device includes a pixel portion and a peripheral portionprovided at a periphery of the pixel portion over a substrate. Aplurality of first wirings and a first common line each extending in afirst direction and being lined up in a second direction intersectingthe first direction over the substrate, a plurality of second wiringsand a second common line each extending in the second direction andbeing lined up in the first direction over the substrate, and aninsulating film formed between the first wiring and the first commonline and the second wiring and the second common line are provided. Athin film transistor and a pixel electrode connected to the thin filmtransistor are included in the pixel portion. One of the first commonline and the second wiring projects into a side of the other of thefirst common line and the second wiring at an intersection of the firstcommon line and the second wiring in the peripheral portion. One of thesecond common line and the first wiring projects into a side of theother of the second common line and the first wiring at an intersectionof the second common line and the first wiring in the peripheral portionof the pixel portion.

A display device includes a pixel portion and a peripheral portionprovided at a periphery of the pixel portion over a substrate. Aplurality of first wirings and a first common line each extending in afirst direction and being lined up in a second direction intersectingthe first direction over the substrate; an insulating film covering thefirst wiring and the first common line; a film formed over at least theinsulating film covering the first wiring and the first common line; anda plurality of second wirings and a second common line each extending inthe second direction and being lined up in the first direction, formedover at least the film are provided. A thin film transistor and a pixelelectrode connected to the thin film transistor are included in thepixel portion. The film formed over at least the insulating filmcovering the first wiring and the first common line is provided with oneof a depressed portion and a separation portion at the intersection ofthe first common line and the second wiring in the peripheral portion.The film formed over at least the insulating film covering the firstwiring and the first common line is provided with one of a depressedportion and a separation portion at the intersection of the secondcommon line and the first wiring in the peripheral portion of the pixelportion.

A display device includes a pixel portion and a peripheral portionprovided at a periphery of the pixel portion over a substrate. Aplurality of first wirings and a first common line each extending in afirst direction and being lined up in a second direction intersectingthe first direction over the substrate, a film formed over at least thefirst wiring and the first common line, an insulating film formed overthe film, and a plurality of second wirings and a second common linewhich are formed over the insulating film and each extend in the seconddirection and are lined up in the first direction are provided. A thinfilm transistor and a pixel electrode connected to the thin filmtransistor are included in the pixel portion. The film formed over atleast the first wiring and the first common line is provided with one ofa depressed portion and a separation portion at the intersection of thefirst common line and the second wiring in the peripheral portion. Thefilm formed over at least the first wiring and the first common line isprovided with one of a depressed portion and a separation portion at theintersection of the second common line and the first wiring in theperipheral portion of the pixel portion.

Note that in the above structure, the film formed over at least theinsulating film covering the first wiring and the first common line orthe film formed over at least the first wiring and the first common lineis an insulating film, a semiconductor film, or a stack of an insulatingfilm and a semiconductor film.

One side of the first common line and the second wiring may be taperedtoward the other of the first common line and the second wiring. Oneside of the second common line and the first wiring may be taperedtoward the other of the second common line and the first wiring.

The first common line may include a plurality of opening portions at theintersection of the first common line and the second wiring. The firstwiring may include a plurality of opening portions at the intersectionof the first wiring and the second common line.

At the intersection of the first common line and the second wiring, theangle that is made by the first common line and the second wiring is notlimited to the case where the first common line and the second wiringoverlap at 90°. In addition, at an intersection of the second commonline and the first wiring, the angle that is made by the second commonline and the first wiring is not limited to the case where the secondcommon line and the first wiring overlap at 90°.

The first common line and the second common line are wirings formed inthe peripheral portion of the pixel portion and are also referred to ascommon potential wirings. The first common line and the second commonline are wirings used to prevent a switching element, typically, a thinfilm transistor from being damaged when excess current flows into theswitching element, typically, the thin film transistor due to dischargecaused by static electricity or the like from the first wiring and thesecond wiring. A signal having constant potential is input from theoutside to the first common line and the second common line.Alternatively, the first common line and the second common line areconnected to ground potential and have fixed potential.

Here, ‘to be lined up’ means that a plurality of elements is arranged ina row.

Here, a depressed portion is a region that is depressed compared to aperipheral region. The bottom of the depressed portion is connected tothe peripheral region. The bottom of the depressed portion does notreach a layer below the object including the depressed portion. Also,the bottom of the depressed portion is not in contact with the layerbelow the object including the depressed portion. In other words, thelayer below the object including the depressed portion is not exposed.To put it differently, the layer below the object including thedepressed portion is not in contact with a layer with which thedepressed portion is filled. Further, the layer below the objectincluding the depressed portion is not in contact with a layer providedinside the depressed portion.

Here, an opening portion is referred to as a void or a portion thatexposes a lower layer through an opening portion. In other words, thelayer below the object including the depressed portion is exposed. Toput it differently, the layer below the object including the depressedportion is in contact with a layer with which the depressed portion isfilled.

The first wiring is a gate wiring. The second wiring is a source wiring.The thin film transistor is formed at the intersection of the firstwiring and the second wiring.

The first wiring and the first common line are formed at the same time;therefore, the first wiring and the first common line are formed overthe same layer. The second wiring and the second common line are formedat the same time; therefore, the second wiring and the second commonline are formed over the same layer. The first wiring and the firstcommon line are formed at the same time; therefore, the first wiring andthe first common line are formed using the same material. The secondwiring and the second common line are formed at the same time;therefore, the second wiring and the second common line are formed usingthe same material.

When one of the first common line and the second wiring projects into aside of the other of the first common line and the second wiring at theintersection of the first common line and the second wiring and highpotential is applied to the second wiring, a dielectric breakdown occursin the insulating film formed between the first common line and thesecond wiring, and current flows into the first common line. Inaddition, when one of the second common line and the first wiringprojects into a side of the other of the second common line and thefirst wiring at the intersection of the second common line and the firstwiring and high potential is applied to the first wiring, a dielectricbreakdown occurs in the insulating film formed between the second commonline and the first wiring, and current flows into the second commonline. As a result, charge electrification and short circuit associatedtherewith in the thin film transistor in the pixel portion and at theintersection of the first wiring and the second wiring can be reduced.

In the above structure, a display device including the first wiringwhich serves as a scan line, the second wiring which serves as a signalline, and a switching element connected to a pixel electrode at theintersection of the scan line and the signal line may be formed. Aprotection circuit is formed between the scan line and the second commonline that is formed at the same time as formation of the signal line. Inaddition, a protection circuit is formed between the signal line and thefirst common line that is formed at the same time as formation of thescan line.

Examples of the display device include a light-emitting device and aliquid crystal display device. The light-emitting device includes alight-emitting element, and the liquid crystal display device includes aliquid crystal element. The light-emitting element includes, in itscategory, an element whose luminance is controlled by current orvoltage, and specifically includes organic electroluminescence (EL) andinorganic electroluminescence (EL).

In addition, the display device includes a panel in which a displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel. The present invention relates toone mode of an element substrate before the display element is completedin a manufacturing process of the display device, and the elementsubstrate is provided with a means for supplying current to the displayelement in each of a plurality of pixels. Specifically, the elementsubstrate may be in a state provided with only a pixel electrode of thedisplay element, a state after a conductive film to be a pixel electrodeis formed and before the conductive film is etched to form the pixelelectrode, or other states.

Note that a display device in this specification means an image displaydevice, a light-emitting device, or a light source (including a lightingdevice). Further, the display device includes any of the followingmodules in its category: a module including a connector such as anflexible printed circuit (FPC), tape automated bonding (TAB) tape, or atape carrier package (TCP); a module having TAB tape or a TCP which isprovided with a printed wiring board at the end thereof; and a modulehaving an integrated circuit (IC) which is directly mounted on a displayelement by a chip on glass (COG) method.

Note that the terms first, second, third, to Nth (N is a natural number)which are used in this specification are used to avoid confusion amongcomponents, and the terms do not limit the components numerically.Spatially relative terms, such as “beneath”, “below”, “lower”, “above”,“upper”, and the like may be used in this specification in order tosimply describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the drawings. It will beunderstood that the spatially relative terms are intended to includedifferent orientations of the device in use or operation as well as theorientation illustrated in the drawings. For example, when the device inthe drawings is turned over, elements described as “below” or “beneath”other elements or features would then be oriented “above” the otherelements or features. Thus, the exemplary term “below” can include bothan orientation of above and below. The device may be otherwise oriented(rotated 90° or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly. As used herein, thesingular forms “a”, “an”, and “the” are intended to also include theplural forms, unless the context clearly indicates otherwise.

By forming the protection circuit at the intersection of two wirings(e.g., the wiring and the common line), an occupied area of theprotection circuit can be reduced. In addition, the reliability of thedisplay device having the switching element can be increased. Further,the frame of the display device can be narrowed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating an element substrate having aprotection circuit of an embodiment mode.

FIGS. 2A and 2B are a cross-sectional view and a plan view eachillustrating a protection circuit of an embodiment mode.

FIGS. 3A to 3C are a cross-sectional view and plan views eachillustrating a protection circuit of an embodiment mode.

FIGS. 4A to 4D are a cross-sectional view and plan views eachillustrating a protection circuit of an embodiment mode.

FIGS. 5A to 5D are cross-sectional views and a plan view illustrating amanufacturing process of a protection circuit of an embodiment mode.

FIGS. 6A to 6D are cross-sectional views and a plan view illustrating amanufacturing process of a protection circuit of an embodiment mode.

FIGS. 7A to 7C are a cross-sectional view and plan views illustrating aprotection circuit of an embodiment mode.

FIG. 8 is a cross-sectional view illustrating a protection circuit of anembodiment mode.

FIG. 9 is a cross-sectional view illustrating a protection circuit of anembodiment mode.

FIGS. 10A to 10C are a cross-sectional view and plan views eachillustrating a protection circuit of an embodiment mode.

FIGS. 11A to 11C are a cross-sectional view and plan views eachillustrating a protection circuit of an embodiment mode.

FIGS. 12A to 12C are a cross-sectional view and plan views eachillustrating a protection circuit of an embodiment mode.

FIGS. 13A to 13F are cross-sectional views illustrating a manufacturingprocess of a protection circuit of an embodiment mode.

FIG. 14 is a plan view illustrating a manufacturing process of aprotection circuit of an embodiment mode.

FIGS. 15A to 15E are cross-sectional views illustrating a manufacturingprocess of a protection circuit of an embodiment mode.

FIG. 16 is a plan view illustrating a manufacturing process of aprotection circuit of an embodiment mode.

FIGS. 17A to 17D are cross-sectional views illustrating a manufacturingprocess of a protection circuit of an embodiment mode.

FIGS. 18A to 18F are cross-sectional views illustrating a manufacturingprocess of a protection circuit of an embodiment mode.

FIGS. 19A to 19E are cross-sectional views illustrating a manufacturingprocess of a protection circuit of an embodiment mode.

FIGS. 20A to 20D are diagrams illustrating a multi-tone photomaskapplicable to an embodiment mode.

FIG. 21 is a plan view illustrating a manufacturing process of aprotection circuit of an embodiment mode.

FIGS. 22A to 22C are perspective views each illustrating a display panelof an embodiment mode.

FIGS. 23A to 23D are perspective views each illustrating an electronicdevice using a display device of an embodiment mode.

FIG. 24 is a diagram illustrating an electronic device using a displaydevice of an embodiment mode.

FIGS. 25A to 25C are perspective views each illustrating an electronicdevice using a display device of an embodiment mode.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes

Hereinafter, embodiment modes will be described with reference to thedrawings. Note that the embodiment modes are not limited to thefollowing description, and it is easily understood by those skilled inthe art that the modes and details disclosed herein can be modified invarious ways without departing from the spirit and scope of theembodiment modes. Thus, the embodiment modes should not be taken asbeing limited to the following description of the embodiment modes. Notethat the same reference numeral is commonly used to denote the samecomponent among the different drawings in the structure of theembodiment modes described below.

(Embodiment Mode 1)

Here, a structure of a protection circuit is described. Note that in anelement substrate 1000 illustrated in FIG. 1, a plurality of firstwirings which extends in a first direction and is lined up in a seconddirection intersecting the first direction is referred to as scan lines,while a plurality of second wirings which extends in the seconddirection and is lined up in the first direction is referred to assignal lines. In a pixel provided in a pixel portion, the scan line isconnected to a gate electrode in the case of an active matrix displaydevice provided with a switching element that controls potential of apixel electrode. Alternatively, part of the scan line functions as agate electrode. Therefore, the first wiring is referred to as a gatewiring 1301 hereinafter. In addition, since the signal line is connectedto a source of the switching element, the second wiring is referred toas a source wiring 1302 hereinafter. However, when the second wiring isconnected to a drain of the switching element, the second wiring can bereferred to as a drain wiring.

In this embodiment mode, in the element substrate 1000 illustrated inFIG. 1, a plurality of gate wirings 1301, a plurality of source wirings1302 intersecting the plurality of gate wirings, a switching elementprovided at each intersection of the gate wirings 1301 and the sourcewirings 1302, and a pixel electrode connected to the switching elementare formed. In addition, a pixel 1303 is formed using the switchingelement and the pixel electrode. An area where the pixels are arrangedin matrix is a pixel portion 1300. The end portion of the gate wiring1301 is provided with a gate wiring input terminal portion 1301 a, andan end portion of the source wiring 1302 is provided with a sourcewiring input terminal portion 1302 a. A first common line 1311 which islined up along the gate wiring 1301 and intersects the source wiringwith an insulating film interposed therebetween, and a second commonline 1312 which is lined up along the source wiring 1302 and intersectsthe gate wiring with the insulating film interposed therebetween areformed between the gate wiring input terminal portion 1301 a and thesource wiring input terminal portion 1302 a and the pixel portion. Theend portion of the first common line 1311 is provided with a firstcommon line input terminal portion 1300 a (also simply referred to as aterminal portion 1300 a), and the end portion of the second common line1312 is provided with a second common line input terminal portion 1300 b(also simply referred to as a terminal portion 1300 a). In addition, thepixel is provided with a liquid crystal element or a light-emittingelement. An IC (a semiconductor integrated circuit) or an FPC for inputof a signal into the gate wiring is connected to the gate wiring inputterminal portion 1301 a. In addition, an IC (a semiconductor integratedcircuit) or an FPC for input of a signal into the source wiring isconnected to the source wiring input terminal portion 1302 a. The firstcommon line input terminal portion 1300 a and the second common lineinput terminal portion 1300 b are connected to respective FPCs, andconstant potential is input to the first common line input terminalportion 1300 a and the second common line input terminal portion 1300 b.Further, current that flows into the first wiring and the second wiringis input to the FPCs. Note that the first common line input terminalportion 1300 a and the second common line input terminal portion 1300 bmay be connected to ground potential and have fixed potential.

In a peripheral portion of the pixel portion 1300, a protection circuit1313 is provided at each intersection of the gate wirings 1301 and thesecond common line 1312, and a protection circuit 1314 is provided ateach intersection of the source wirings 1302 and the first common line1311.

Next, a structure of the protection circuit 1313 is described.

FIGS. 2A and 2B are a cross-sectional view and a plan view of theprotection circuit 1313 described in this embodiment mode. FIG. 2A is across-sectional view taken along line A-B in the plan view of theintersection of the gate wiring 1301 and the second common line 1312illustrated in FIG. 2B.

As illustrated in FIG. 2A, the gate wiring 1301 is formed over asubstrate 1001, and an insulating film 1322 is formed to cover the gatewiring 1301. In addition, the insulating film 1322 is provided with adepressed portion 1325, which overlaps the gate wiring 1301. The secondcommon line 1312 is formed over the insulating film 1322. The depressedportion 1325 in the insulating film 1322 is filled with a material thatforms the second common line 1312; therefore, the second common line1312 projects into the gate wiring 1301 side. In other words, thematerial that forms the second common line 1312 is provided inside thedepressed portion 1325 in the insulating film 1322; therefore, thesecond common line 1312 projects into the gate wiring 1301 side. Theprotection circuit 1313 is formed using the gate wiring 1301, theinsulating film 1322 having the depressed portion, and the second commonline 1312.

When high voltage is applied to the gate wiring 1301, a dielectricbreakdown occurs between the gate wiring 1301 and the second common line1312, and current flows into the second common line 1312 in theprotection circuit 1313 described in this embodiment mode. As a result,charge electrification and short circuit associated therewith in a thinfilm transistor in the pixel portion and at the intersection of the gatewiring 1301 and the source wiring 1302 can be reduced. Since theprotection circuit is formed at the intersection of the gate wiring 1301and the second common line 1312, an occupied area of the protectioncircuit in a plane can be reduced compared with a protection circuit inwhich discharge occurs between conductive films that face each other ina plane.

At this time, the second common line 1312 is preferably tapered towardthe gate wiring 1301 side. That is, it is preferable that the depressedportion in the insulating film 1322 be a cone, a truncated cone, apyramid with the polygon as its base, or a truncated polygonal pyramidwhich has an apex on the gate wiring 1301 side. When the second commonline 1312 is formed in the depressed portion having such a shape, adielectric breakdown easily occurs between the gate wiring 1301 and thesecond common line 1312, and charge electrification and short circuitassociated therewith in the thin film transistor in the pixel portionand at the intersection of the gate wiring 1301 and the source wiring1302 can be reduced.

As the substrate 1001, a plastic substrate having heat resistance thatcan withstand a processing temperature of this manufacturing process orthe like as well as a non-alkaline glass substrate manufactured by afusion method or a float method such as a substrate of bariumborosilicate glass, aluminoborosilicate glass, or aluminosilicate glass,or a ceramic substrate can be used. Alternatively, a metal substratesuch as a stainless steel alloy substrate, provided with an insulatingfilm over its surface, may also be used.

The gate wiring 1301 and the first common line 1311 are formed using ametal material. As the metal material, aluminum, chromium, titanium,tantalum, molybdenum, copper, or the like is applied. For example, thegate wiring 1301 and the first common line 1311 are preferably formedusing aluminum or a stacked structure of aluminum and a barrier metal.As the barrier metal, a refractory metal such as titanium, molybdenum,or chromium is applied. The barrier metal is preferably provided forpreventing hillocks and oxidation of aluminum.

The gate wiring 1301 and the first common line 1311 are formed to have athickness of equal to or more than 50 nm and equal to or less than 300nm. The gate wiring 1301 and the first common line 1311 may be formed tohave a thickness of equal to or more than 50 nm and equal to or lessthan 100 nm, whereby disconnection of a microcrystalline semiconductorfilm and a wiring which are to be formed later can be prevented.Further, the gate wiring 1301 and the first common line 1311 may beformed to have a thickness of equal to or more than 150 nm and equal toor less than 300 nm, whereby the resistivity of the gate wiring 1301 andthe first common line 1311 can be reduced and an area can be increased.

Note that because a semiconductor film and a wiring are formed over thegate wiring 1301 and the first common line 1311, it is preferable thatthe gate wiring 1301 and the first common line 1311 be processed so thatthe end portions thereof are tapered in order to prevent disconnection.Further, although not illustrated, a capacitor wiring can also be formedin this step.

The insulating film 1322 can be formed using a silicon oxide film; asilicon nitride film; a silicon oxynitride film; a silicon nitride oxidefilm; an aluminum nitride film; another inorganic insulating film; anorganic insulating film of polyimide, an epoxy resin, an acrylic resin,or the like. Here, the insulating film 1322 is formed with a singlelayer; however, it may have a stacked structure.

The second common line 1312 and the source wiring 1302 are preferablyformed with a single layer or stacked layers using aluminum; copper; oran aluminum alloy to which a migration prevention metal, an element forimproving heat resistance property, or an element for preventinghillocks, such as copper, silicon, titanium, neodymium, scandium, ormolybdenum is added. Alternatively, a film formed using titanium,tantalum, molybdenum, tungsten, or a nitride of any of these elementsand an aluminum film or an aluminum alloy film may be formed in thisorder on the insulating film 1322 side of each of the second common line1312 and the source wiring 1302. Further alternatively, top and bottomsurfaces of aluminum or an aluminum alloy may be each covered withtitanium, tantalum, molybdenum, tungsten, or a nitride of any of theseelements to form a stacked structure.

As illustrated in FIGS. 3A to 3C, a gate wiring 1331 may have aplurality of opening portions at an intersection with the second commonline 1312. FIG. 3B illustrates a mode in which two rectangular openingportions 1332 a are provided in a gate wiring 1331 a, and FIG. 3Cillustrates a mode in which two polygonal opening portions 1332 b areprovided in a gate wiring 1331 b. When the opening portions are providedin the gate wiring 1331, the width of the gate wiring in a regionbetween the opening portions is small, whereby the gate wiring has aprojection shape. When a side surface of the gate wiring 1331 is formedto have a slant, as illustrated in FIG. 3A, the cross section of thegate wiring 1331 in the region between the opening portions has aprojection shape tapered toward the second common line 1312 side.Therefore, the gate wiring 1331 and the second common line 1312 projecteach other with an insulating film interposed therebetween, whereby adielectric breakdown easily occurs. Thus, charge electrification andshort circuit associated therewith in the thin film transistor in thepixel portion and at the intersection of the gate wiring 1331 and thesource wiring 1302 can be reduced.

In FIGS. 2A and 2B and FIGS. 3A to 3C, in a region where the gatewirings 1301 and 1331 and the second common line 1312 overlap eachother, the insulating film having the depressed portion is formed.However, in FIGS. 2A and 2B, and FIGS. 3A to 3C, a first insulating filmand a second insulating film are formed in a region where the gatewiring 1301 and the second common line 1312 overlap each other. One ofthe first insulating film and the second insulating film is separatedinto a plurality of films, and there is a separation portion 1328 abetween the separated films, as illustrated in FIGS. 4A to 4D. In theseparation portion 1328 a, the second common line 1312 can project intothe gate wiring 1301 side.

As illustrated in FIG. 4A, the gate wiring 1301 is formed over thesubstrate 1001 and a first insulating film 1323 is formed over the gatewiring 1301. In a region which is located over the first insulating film1323 and overlaps part of the gate wiring 1301, second insulating films1327 a and 1327 b are separated, and there is the separation portion1328 a between the second insulating films 1327 a and 1327 b. That is,the second insulating films 1327 a and 1327 b are provided with aconstant distance above the gate wiring 1301. In addition, the secondcommon line 1312 is formed over the first insulating film 1323, and thesecond insulating films 1327 a and 1327 b. The second common line 1312is in contact with side surfaces of the separated second insulatingfilms 1327 a and 1327 b. The second common line 1312 projects into thegate wiring 1301 side in the separation portion 1328 a where the secondinsulating films 1327 a and 1327 b are separated from each other.Therefore, when high voltage is applied to the gate wiring 1301 due tostatic electricity or the like via the gate wiring input terminalportion 1301 a, or when static electricity is generated in the switchingelement, a dielectric breakdown occurs between the gate wiring 1301 andthe second common line 1312, and current flows into the second commonline 1312. As a result, charge electrification and short circuitassociated therewith in the switching element in the pixel portion andat the intersection of the gate wiring 1301 and the source wiring 1302can be reduced.

It is preferable that side surfaces of the second insulating films 1327a and 1327 b have a slant. As a result, the separation portion 1328 a istapered toward the gate wiring 1301 side. Therefore, when high voltageis applied to the gate wiring 1301, a dielectric breakdown easily occursin the gate wiring 1301 and the second common line 1312, and shortcircuit at the intersection of the gate wiring 1301 and the sourcewiring and in the thin film transistor can be reduced.

Here, the case is described in which the second insulating films areseparated, and the gate wiring and the second common line are separatedfrom each other using the first insulating film. However, the case maybe used in which the first insulating film is separated and the gatewiring and the second common line are insulated using the secondinsulating film.

The first insulating film 1323 or the second insulating films 1327 a and1327 b can be formed using the inorganic insulating film and the organicinsulating film which are used for an insulating film 1322, asappropriate. Note that as illustrated in FIGS. 4A to 4D, when theseparated second insulating films are formed over the first insulatingfilm, it is preferable that a material of each insulating film beselected so that the second insulating films are etched selectively. Asan example, the first insulating film can be formed using an inorganicinsulating film, and the second insulating films can be formed usingorganic insulating films.

Note that in FIGS. 4A and 4B, the insulating film is separated in thedirection along which the gate wiring extends (the separation portion1328 a). Instead of this, as illustrated in FIG. 4C, the insulating filmmay be separated in the direction along which the second common line1312 extends (a separation portion 1328 b). At this time, insulatinglayers 1327 c and 1327 d are formed. Further, as illustrated in FIG. 4D,the insulating film may be separated in both a direction along which thegate wiring 1301 extends and a direction along which the second commonline 1312 extends (a separation portion 1328 c). In this case, across-shaped separation portion is formed. In addition, insulatinglayers 1327 e to 1327 h are formed.

Note that here, the protection circuit 1313 is formed in the regionwhere the gate wiring 1301 and the second common line 1312 overlap eachother. The protection circuit 1314 formed in a region where the firstcommon line 1311 and the source wiring 1302 overlap each other also hasa shape similar to the structure of the protection circuit illustratedin FIGS. 2A and 2B and FIGS. 3A to 3C. In this case, the gate wirings1301 and 1331 illustrated in FIGS. 2A and 2B, FIGS. 3A to 3C, and FIGS.4A to 4D may be the first common line 1311, and the second common line1312 illustrated in FIGS. 2A and 2B, FIGS. 3A to 3C, and FIGS. 4A to 4Dmay be the source wiring 1302. As a result, when the insulating filmhaving a depressed portion is formed between the first common line 1311and the source wiring 1302, and the source wiring is formed in thedepressed portion, whereby a dielectric breakdown between the firstcommon line 1311 and the source wiring 1302 easily occurs. Thus, chargeelectrification and short circuit associated therewith in the switchingelement in the pixel portion and at the intersection of the gate wiring1301 and the source wiring 1302 can be reduced.

Note that this embodiment mode is described based on a mode of an activematrix display device provided with the switching element which controlspotential of the pixel electrode in the pixel provided in the pixelportion. However, the protection circuit described in this embodimentmode can be provided in a mode of a passive matrix display devicewithout a switching element in a pixel. In this case, a plurality offirst wirings which extends in the first direction and is lined up inthe second direction intersecting with the first direction can bereferred to as one of a row wiring and a column wiring, while aplurality of second wirings which extends in the second direction and islined up in the first direction can be referred to as the other of therow wiring and the column wiring.

The protection circuit of this embodiment mode is a protection circuithaving a structure in which a dielectric breakdown can occur indirections above and below the substrate, that is, in the film thicknessdirection of a film formed over the substrate. Thus, the occupied areaof the protection circuit can be reduced.

(Embodiment Mode 2)

In this embodiment mode, a method of manufacturing the protectioncircuit illustrated in FIGS. 2A and 2B and FIGS. 4A to 4D described inEmbodiment Mode 1 will be described with reference to FIGS. 5A to 5D andFIGS. 6A to 6D.

FIG. 5D is a plan view at the intersection of the gate wiring 1301 andthe second common line 1312, and FIG. 5C illustrates a cross-sectionalview taken along line A-B of FIG. 5D.

As illustrated in FIG. 5A, the gate wiring 1301 is formed over thesubstrate 1001. Next, the first insulating film 1323 is formed over thegate wiring 1301. Then, the second insulating film 1324 is formed overthe first insulating film 1323. The first insulating film 1323 and thesecond insulating film 1324 may be formed using an insulating material,as appropriate. However, it is preferable that a combination in whichonly the second insulating film 1324 can be selectively etched be used.For example, the first insulating film 1323 is formed using an inorganicinsulating film, and the second insulating film 1324 is formed using aphotosensitive resin. In this case, as the inorganic insulating film,there are silicon nitride, silicon oxide, silicon nitride oxide, siliconoxynitride, aluminum nitride, aluminum oxynitride, and the like. As thephotosensitive resin, there are an acrylic resin, polyimide, siloxanepolymer, and the like. With such a combination, the second insulatingfilm 1324 is exposed to light to have a desired shape, and then thesecond insulating film 1324 is developed, whereby a depressed portionwhich exposes the first insulating film 1323 can be formed.Alternatively, the first insulating film 1323 may be formed using aninorganic insulating film, and the second insulating film 1324 may beformed using a non-photosensitive resin such as an acrylic resin,polyimide, or siloxane polymer. As a result, the second insulating film1324 can be selectively subjected to dry etching or wet etching.

Next, as illustrated in FIG. 5B, the second insulating film 1324 isselectively etched, and the depressed portion 1325 which exposes thefirst insulating film 1323 is formed. That is, a second insulating film1326 having an opening portion is formed.

Next, as illustrated in FIG. 5C, the second common line 1312 is formedover the exposed portion of the first insulating film 1323 and thesecond insulating film 1326. As a result, the second common line 1312which projects into the gate wiring 1301 side can be formed. Since thesecond common line 1312 projects into the gate wiring 1301 side, adielectric breakdown can occur at the intersection of the gate wiring1301 and the second common line 1312. Therefore, even if high voltage isapplied to the gate wiring 1301, current can flow into the second commonline 1312, whereby short circuit in the switching element in the pixelportion or at the intersection of wirings can be reduced.

Note that in FIG. 5D, the top surface of the opening portion has acircular shape; however, an oval, a polygon, or the like can be used.

Next, a method of manufacturing the protection circuit illustrated inFIGS. 4A to 4D is described with reference to FIGS. 6A to 6D. FIGS. 6Ato 6D are different from FIGS. 5A to 5D in that the second insulatingfilms 1327 a and 1327 b are separated from each other.

FIG. 6C is a cross-sectional view taken along line A-B of FIG. 6D.

As illustrated in FIG. 6A, the gate wiring 1301 is formed over thesubstrate 1001. Next, the first insulating film 1323 is formed over thegate wiring 1301. Then, the second insulating film 1324 is formed overthe first insulating film 1323.

Next, as illustrated in FIG. 6B, the second insulating film 1324 isselectively etched, and the second insulating films 1327 a and 1327 bwhich are separated from each other are formed. The second insulatingfilms 1327 a and 1327 b are separated from each other, and there is theseparation portion 1328 a over the gate wiring 1301 between the secondinsulating films 1327 a and 1327 b. Note that here, part of the secondinsulating film 1324 is removed, and the second insulating films 1327 aand 1327 b are formed; however, a composition may be selectivelydischarged by a droplet discharge method (ink-jet method) and burned toform the second insulating films 1327 a and 1327 b.

Next, as illustrated in FIG. 6C, the second common line 1312 is formedover the exposed portion of the first insulating film 1323 and thesecond insulating films 1327 a and 1327 b. As a result, the secondcommon line 1312 which projects into the gate wiring 1301 side can beformed. Since the second common line 1312 projects into the gate wiring1301 side, a dielectric breakdown can occur at the intersection of thegate wiring 1301 and the second common line 1312. Therefore, even ifhigh voltage is applied to the gate wiring 1301, current can flow intothe second common line 1312, whereby short circuit in the thin filmtransistor in the pixel portion or at the intersection of wirings can bereduced.

Note that in FIGS. 6A to 6D, the insulating film is separated in thedirection along which the gate wiring extends (the separation portion1328 a). Instead of this, as illustrated in FIG. 4C, the insulating filmmay be separated in the direction along which the second common line1312 extends (the separation portion 1328 b). Further alternatively, asillustrated in FIG. 4D, the insulating film may be separated in both thedirection along which the gate wiring 1301 extends and the directionalong which the second common line 1312 extends (the separation portion1328 c). In this case, a cross-shaped separation portion is formed.

Here, the protection circuit is formed in the region where the gatewiring 1301 and the second common line 1312 overlap each other. Theprotection circuit formed in the region where the first common line 1311and the source wiring 1302 overlap each other also has a shape similarto the structure of the protection circuit illustrated in FIGS. 2A and2B and FIGS. 3A to 3C. In this case, the gate wiring 1301 illustrated inFIGS. 5A to 5D and FIGS. 6A to 6D may be the first common line 1311, andthe second common line 1312 illustrated in FIGS. 5A to 5D and FIGS. 6Ato 6D may be the source wiring 1302. As a result, when the insulatingfilm having a depressed portion is formed between the first common line1311 and the source wiring 1302, and when the source wiring is formed inthe depressed portion, a dielectric breakdown between the first commonline 1311 and the source wiring 1302 easily occurs. Thus, chargeelectrification and short circuit associated therewith in the thin filmtransistor in the pixel portion and at the intersection of the gatewiring 1301 and the source wiring 1302 can be reduced.

Through the above steps, the protection circuit can be formed in which adielectric breakdown can occur in directions above and below thesubstrate, that is, in the film thickness direction of a film formedover the substrate. As a result, a protection circuit with a smalloccupied area can be formed.

(Embodiment Mode 3)

In this embodiment mode, a structure of a protection circuit will bedescribed with reference to FIGS. 7A to 7C, FIG. 8, FIG. 9, FIGS. 10A to10C, FIGS. 11A to 11C, and FIGS. 12A to 12C. In the protection circuit,a semiconductor film that has a depressed portion or is separated isformed above a gate wiring; a second common line is formed above thesemiconductor film; and an insulating film that insulates the gatewiring and the second common line is formed between the gate wiring andthe second common line.

FIGS. 7B and 7C are plan views at the intersection of the gate wiring1301 and the second common line 1312, and FIG. 7A illustrates across-sectional view taken along line C-D of FIG. 7B.

The gate wiring 1301 is formed over the substrate 1001, and insulatingfilms 1341 a and 1341 b are formed to cover the gate wiring 1301. Overthe insulating film 1341 b, separated stacks of semiconductor films areformed. Here, a stack of an amorphous semiconductor film 1344 and asemiconductor film 1346 to which an impurity element imparting oneconductivity type is added, and a stack of an amorphous semiconductorfilm 1345 and a semiconductor film 1347 to which an impurity elementimparting one conductivity type is added are separated from each other(a separation portion 1348). In addition, the separation portion 1348 isformed over the gate wiring 1301. Note that the shapes of the separationportions 1328 a to 1328 c illustrated in FIGS. 4B to 4D described inEmbodiment Mode 1 can be applied to the shape of the separation portion1348.

Second common lines 1312 a to 1312 c are formed over the stacks of thesemiconductor films and the insulating film 1341 b. The separationportion 1348 in which the stacks of the semiconductor films areseparated is filled with a material that forms the second common lines1312 a to 1312 c, whereby the second common lines 1312 a to 1312 cproject into the gate wiring 1301 side. Further, the gate wiring 1301and the second common lines 1312 a to 1312 c are insulated from eachother by the insulating films 1341 a and 1341 b.

The material which is used for the insulating film 1322 described inEmbodiment Mode 1 can be used for the insulating films 1341 a and 1341b, as appropriate.

When a thin film transistor or a diode is formed as a switching elementof a pixel electrode formed in a pixel over a substrate, the amorphoussemiconductor films 1344 and 1345 can be formed at the same time asformation of an amorphous semiconductor film of the thin film transistoror the diode. The amorphous semiconductor films 1344 and 1345 are formedusing an amorphous silicon film, an amorphous silicon germanium film, anamorphous germanium film, or the like.

The semiconductor films 1346 and 1347 to which an impurity elementimparting one conductivity type is added can be formed as the switchingelement of the pixel electrode formed in the pixel over the substrate,at the same time as formation of a source region and drain region of athin film transistor. In the case where an n-channel thin filmtransistor is formed, phosphorus may be added as a typical impurityelement to the semiconductor films 1346 and 1347 to which an impurityelement imparting one conductivity type is added, and an impurity gassuch as PH₃ may be added to silicon hydride. In addition, in the casewhere a p-channel thin film transistor is formed, boron may be added asa typical impurity element, and an impurity gas such as B₂H₆ may beadded to silicon hydride. When phosphorus or boron concentration is setat 1×10¹⁹ atoms/cm³ to 1×10²¹ atoms/cm³, ohmic contact with the secondcommon lines 1312 a to 1312 c can be made. The pair of the semiconductorfilms 1346 and 1347 to which an impurity element imparting oneconductivity type is added can be formed using a microcrystallinesemiconductor or an amorphous semiconductor. The semiconductor films1346 and 1347 to which an impurity element imparting one conductivitytype is added are formed to a thickness of equal to or more than 2 nmand equal to or less than 50 nm. By reduction in the thicknesses of thepair of the semiconductor films to which an impurity element impartingone conductivity type is added, throughput can be improved. Note thatwhen a diode is used as the switching element, an n-type semiconductorfilm to which phosphorus is added or a p-type semiconductor film towhich boron is added can be formed as the semiconductor films 1346 and1347 to which an impurity element imparting one conductivity type isadded.

Note that in FIG. 7B, the protection circuit is formed in the separationportion 1348; however, stacks of semiconductor films may be providedwith an opening portion 1349 as illustrated in FIG. 7C, and the openingportion 1349 may be filled with the material that forms the secondcommon lines 1312 a to 1312 c. As a result, the second common lines 1312a to 1312 c project into the gate wiring 1301 side in the openingportion 1349 of the stacks of semiconductor films, whereby a protectioncircuit can be formed.

The second common lines 1312 a to 1312 c project into the gate wiring1301 side in the separation portion 1348 or the opening portion 1349.Therefore, when high voltage is applied to the gate wiring 1301, adielectric breakdown occurs between the gate wiring 1301 and the secondcommon line 1312, and current generated at this time flows into thesecond common line 1312. As a result, charge electrification and shortcircuit associated therewith in the switching element in the pixelportion and at the intersection of the gate wiring 1301 and the sourcewiring 1302 can be reduced.

When a thin film transistor or a diode is used as the switching elementin the pixel portion, a protection circuit can be formed at the sametime as formation of the thin film transistor or the diode; accordingly,a display device can be formed through a small number of steps and withhigh yield.

A protection circuit having a stacked structure of stacks ofsemiconductor films, which is different from that in FIGS. 7A to 7C, isdescribed with reference to FIG. 8.

A protection circuit illustrated in FIG. 8 is different from that inFIGS. 7A to 7C in that stacks of semiconductor films which are formedbetween the insulating film 1341 b and the second common lines 1312 a to1312 c are formed using a stack including a microcrystallinesemiconductor film 1351, an amorphous semiconductor film 1353, and thesemiconductor film 1346 to which an impurity element imparting oneconductivity type is added, and a stack including a microcrystallinesemiconductor film 1352, an amorphous semiconductor film 1354, and thesemiconductor film 1347 to which an impurity element imparting oneconductivity type is added. Note that here, the mode in which there isthe separation portion 1348 between the stacks is described; however,instead of the separated stacks of the semiconductor films, stacks ofsemiconductor films having an opening portion may be formed asillustrated in FIG. 7C.

The microcrystalline semiconductor films 1351 and 1352 are formed usinga microcrystalline silicon film, a microcrystalline silicon germaniumfilm, a microcrystalline germanium film, or the like.

The microcrystalline semiconductor film is a film including asemiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystal) structures. Thissemiconductor is a semiconductor which has a third state that is stablein terms of free energy, and is a crystalline semiconductor which hasshort-range order and lattice distortion, and column-like or needle-likecrystals with a grain size of 0.5 nm to 20 nm grown in the direction ofa normal line with respect to the surface of the substrate. Further,there is a non-single-crystal semiconductor between a plurality ofmicrocrystalline semiconductors. The Raman spectrum of microcrystallinesilicon, which is a typical example of a microcrystalline semiconductor,is located in lower wave numbers than 520 cm⁻¹, which represents a peakof the Raman spectrum of single crystal silicon. That is, the peak ofthe Raman spectrum of the microcrystalline silicon exists between 520cm⁻¹ which represents single crystal silicon and 480 cm⁻¹ whichrepresents amorphous silicon. The microcrystalline silicon includeshydrogen or halogen of at least 1 at. % to terminate dangling bonds.Moreover, a noble gas element such as helium, argon, krypton, or neonmay be included to further promote lattice distortion, so that stabilityis enhanced and a favorable microcrystalline semiconductor film can beobtained. Such description about a microcrystalline semiconductor filmis disclosed in, for example, U.S. Pat. No. 4,409,134.

The microcrystalline semiconductor films 1351 and 1352 are formed with athickness of more than or equal to 5 nm and less than or equal to 200nm, preferably more than or equal to 5 nm and less than or equal to 100nm, more preferably more than or equal to 10 nm and less than or equalto 50 nm, still more preferably more than or equal to 10 nm and lessthan or equal to 25 nm.

Further, it is preferable that the concentration of oxygen and theconcentration of nitrogen in the microcrystalline semiconductor films1351 and 1352 each be typically lower than 3×10¹⁹ atoms/cm³, preferablylower than 3×10¹⁸ atoms/cm³; and that the concentration of carbon beequal to or less than 3×10¹⁸ atoms/cm³. Lower concentration of oxygen,nitrogen, or carbon mixed in the microcrystalline semiconductor filmscan suppress generation of defects in the microcrystalline semiconductorfilms 1351 and 1352. Furthermore, oxygen or nitrogen in themicrocrystalline semiconductor films hinders crystallization. Therefore,the microcrystalline semiconductor films 1351 and 1352 include oxygenand nitrogen at relatively low concentrations, whereby the crystallinityof the microcrystalline semiconductor films 1351 and 1352 can beimproved.

By adding an impurity element which serves as an acceptor to themicrocrystalline semiconductor films 1351 and 1352 at the same time asor after formation of the microcrystalline semiconductor films 1351 and1352, the threshold value can be controlled. A typical example of theimpurity element which serves as an acceptor is boron, and an impuritygas such as B₂H₆ or BF₃ is preferably mixed into silicon hydride at from1 ppm to 1000 ppm, preferably from 1 ppm to 100 ppm. Further, theconcentration of boron is preferably set to be approximately one tenththat of the impurity element which serves as a donor, e.g., from 1×10¹⁴atoms/cm³ to 6×10¹⁶ atoms/cm³.

The amorphous semiconductor films 1353 and 1354 are formed using anamorphous semiconductor film. Alternatively, an amorphous semiconductorfilm which includes a halogen such as fluorine or chlorine is used. Theamorphous semiconductor films 1353 and 1354 each have a thickness of 50nm to 200 nm. Examples of the amorphous semiconductor film are anamorphous silicon film, an amorphous silicon film including germanium,and the like.

When the amorphous semiconductor films 1353 and 1354 are in contact withthe microcrystalline semiconductor films 1351 and 1352, the oxidation ofthe microcrystalline semiconductor films can be reduced.

Through the above steps, the second common lines 1312 a to 1312 cproject into the gate wiring 1301 side in the separation portion or thedepressed portion of the stacks of the semiconductor films, whereby aprotection circuit can be formed.

Next, a protection circuit having a stacked structure of stacks ofsemiconductor films, which is different from those in FIGS. 7A to 7C andFIG. 8, is described with reference to FIG. 9.

A protection circuit illustrated in FIG. 9 is different from those inFIGS. 7A to 7C and FIG. 8 in that stacks of semiconductor films whichare formed between the insulating film 1341 b and the second commonlines 1312 a to 1312 c are formed using a stack including amicrocrystalline semiconductor film 1361 to which an impurity elementthat serves as a donor is added, an amorphous semiconductor film 1363,an amorphous semiconductor film 1365, and a semiconductor film 1367 towhich an impurity element imparting one conductivity type is added, anda stack including a microcrystalline semiconductor film 1362 to which animpurity element that serves as a donor is added, an amorphoussemiconductor film 1364, an amorphous semiconductor film 1366, and asemiconductor film 1368 to which an impurity element imparting oneconductivity type is added. Note that here, the mode in which the stacksare separated (the separation portion 1348) is described; however,instead of the separated stacks of the semiconductor films, stacks ofsemiconductor films having an opening portion may be formed asillustrated in FIG. 7C.

The microcrystalline semiconductor films 1361 and 1362 to which animpurity element that serves as a donor is added are formed using themicrocrystalline semiconductor films 1351 and 1352 illustrated in FIG. 8to which an impurity element that serves as a donor is added. Further,examples of the impurity element which serves as a donor are phosphorus,arsenic, antimony, and the like.

The concentration of the impurity element which serves as a donor ismore than or equal to 6×10¹⁵ atoms/cm³ and less than or equal to 3×10¹⁸atoms/cm³, preferably more than or equal to 1×10¹⁶ atoms/cm³ and lessthan or equal to 3×10¹⁸ atoms/cm³, more preferably more than or equal to3×10¹⁶ atoms/cm³ and less than or equal to 3×10¹⁷ atoms/cm³. Theconcentration of the impurity element which serves as a donor and isincluded in the microcrystalline semiconductor films is set to be in theabove range, whereby crystallinity of the microcrystalline semiconductorfilms at the interface between the insulating film 1341 b and themicrocrystalline semiconductor films 1361 and 1362 to which an impurityelement that serves as a donor is added can be improved, and resistanceof the microcrystalline semiconductor films 1361 and 1362 to which animpurity element that serves as a donor is added can be reduced.

The side surfaces of the microcrystalline semiconductor films 1361 and1362 to which an impurity element that serves as a donor is added in aregion which is in contact with the separation portion 1348 are incontact with the second common lines 1312 a to 1312 c; however, sidesurfaces other than the region are covered with the amorphoussemiconductor films 1365 and 1366.

Through the above steps, the second common lines 1312 a to 1312 cproject into the gate wiring 1301 side in the separation portion 1348 orthe depressed portion of the stacks of the semiconductor films, wherebya protection circuit can be formed.

A cross-sectional structure and planar structure of a protection circuitwhich can be applied to FIGS. 7A to 7C, FIG. 8, and FIG. 9 are describedwith reference to FIGS. 10A to 10C. Here, the cross-sectional structureand the planar structure are described based on the structure of FIGS.7A to 7C in which the stack including the amorphous semiconductor film1344 and the semiconductor film 1346 to which an impurity elementimparting one conductivity type is added, and the stack including theamorphous semiconductor film 1345 and the semiconductor film 1347 towhich an impurity element imparting one conductivity type is added areseparated from each other. The cross-sectional structure and the planarstructure are applicable to FIG. 8 and FIG. 9, as appropriate.

FIG. 10A is a cross-sectional view taken along line C-D in a plan viewillustrated in FIG. 10B. As illustrated in FIG 10A, the gate wiring 1301is formed over the substrate 1001; the insulating films 1341 a and 1341b are formed over the gate wiring 1301; and stacks of semiconductorfilms are formed over the insulating film 1341 b. In addition, aninsulating film 1372 is formed over the stacks of the semiconductorfilms and the insulating film 1341 b. Further, the second common lines1312 a to 1312 c are formed over the insulating film 1372. By using theinsulating films 1341 a and 1341 b and the insulating film 1372, thegate wiring 1301 and the second common lines 1312 a to 1312 c areinsulated. In addition, by using the insulating film 1372, the secondcommon lines 1312 a to 1312 c and the stacks of the semiconductor filmsare insulated. The stacks of the semiconductor films are separated, andthe second common lines 1312 a to 1312 c project into the gate wiring1301 side in the separation portion 1348.

Through the above steps, the second common lines 1312 a to 1312 cproject into the gate wiring 1301 side in the separation portion or thedepressed portion of the stacks of the semiconductor films, whereby aprotection circuit can be formed.

Note that in FIG 10B, the second common lines 1312 a to 1312 c projectinto the gate wiring 1301 side in the separation portion 1348 of thestacks of the semiconductor films; however, as illustrated in FIG. 10C,the stacks of the semiconductor films can be provided with the openingportion 1349, and the second common lines 1312 a to 1312 c project intothe gate wiring 1301 side in the region, whereby a protection circuitcan be formed.

In the protection circuit illustrated in FIGS. 10A to 10C, theinsulating films 1341 a and 1341 b formed between the gate wiring 1301and the stacks of the semiconductor films are not necessarily provided.This structure is described with reference to FIGS. 11A to 11C.

FIG. 11A is a cross-sectional view taken along line C-D in a plan viewillustrated in FIG. 11B. FIG. 11A is different from FIG. 10A in thatstacks of semiconductor films are in contact with the gate wiring 1301without an insulating film interposed therebetween. That is, theinsulating film 1372 is formed over the stacks of the semiconductorfilms and the gate wiring 1301. The gate wiring 1301 and the secondcommon lines 1312 a to 1312 c are insulated by the insulating film 1372.In addition, the second common lines 1312 a to 1312 c and the stacks ofthe semiconductor films are insulated by the insulating film 1372.Further, the second common lines 1312 a to 1312 c project into the gatewiring 1301 side in the separation portion 1348 of the stacks of thesemiconductor films, whereby a protection circuit can be formed.

Note that in FIG. 11B, the second common lines 1312 a to 1312 c projectinto a gate wiring 1301 side in the separation portion 1348 of thestacks of the semiconductor films. As illustrated in FIG. 11C, thestacks of the semiconductor films are provided with the opening portion1349, and the second common lines 1312 a to 1312 c project into the gatewiring 1301 side in the region, whereby a protection circuit can beformed.

Next, a structure that can be applied to the protection circuitillustrated in FIGS. 7A to 7C, FIG. 8, FIG. 9, FIGS. 10A to 10C, andFIGS. 11A to 11C is described with reference to FIGS. 12A to 12C. Asillustrated in FIG. 12A, the gate wiring 1371 may have a plurality ofopening portions 1376 at an intersection with the second common lines1312 a to 1312 c. FIG. 12B illustrates a mode in which two rectangularopening portions 1376 are provided in the gate wiring 1371, and FIG. 12Cillustrates a mode in which two polygonal opening portions 1378 areprovided in the gate wiring 1371. When the opening portions are providedin the gate wiring 1371, the width of the gate wiring in a regionbetween the opening portions is small, whereby the gate wiring has aprojection shape. When a side surface of the gate wiring 1371 is formedto have a slant, as illustrated in FIG. 12A, the cross section of thegate wiring 1371 in the region between the opening portions has aprojection shape tapered toward the second common lines 1312 a to 1312 cside. Therefore, the gate wiring 1371 and the second common lines 1312 ato 1312 c project with the insulating films interposed therebetween,whereby a dielectric breakdown easily occurs. Thus, chargeelectrification and short circuit associated therewith in the thin filmtransistor in the pixel portion and at the intersection of the gatewiring 1371 and the source wiring 1302 can be reduced.

Note that here, the gate wiring and the second common lines project in aseparation portion 1377 of the stacks of the semiconductor films.Instead of this, a structure may be used in which a depressed portion isprovided in the stacks of the semiconductor films and the gate wiringand the second common lines project in the region.

(Embodiment Mode 4)

In this embodiment mode, a process of forming the protection circuitdescribed in Embodiment Mode 3 at the same time as formation of a thinfilm transistor will be described hereinafter. Note that a mode in whichthe thin film transistor and the protection circuit are formed using thesame gate wiring is described in this embodiment mode.

A manufacturing process of the protection circuit illustrated in FIG. 8and the thin film transistor is described with reference to FIGS. 13A to13F. Note that a cross section of a region where the protection circuitis formed is illustrated on the left side of FIGS. 13A to 13F, and across section of a region where the thin film transistor is formed as aswitching element in a pixel is illustrated on the right side of FIGS.13A to 13F. In addition, the right side of FIGS. 13A to 13F is also across-sectional view taken along line Q-R illustrated in FIG. 14. Notethat in this embodiment mode, the thin film transistor is described asthe switching element formed in the pixel. Instead of the thin filmtransistor, a diode, a metal-insulator-metal (MIM), or micro electromechanical systems (MEMS) can also be used.

As illustrated in FIG. 13A, a conductive film 1011 is formed over thesubstrate 1001. The conductive film 1011 can be formed using thematerial given as a material for the gate wiring 1301 and the firstcommon line 1311 described in Embodiment Mode 1. The conductive film1011 is formed by a sputtering method, a CVD method, a plating method, aprinting method, a droplet discharge method, or the like.

Then, after a resist is applied to the conductive film 1011, theconductive film 1011 is etched into a desired shape by using a resistmask formed by a photolithography step. Thus, as illustrated in FIG.13B, the gate wiring 1301 is formed. Then, the resist mask is removed.

Next, a gate insulating film 1022 is formed over the gate wiring 1301and the substrate 1001. The gate insulating film 1022 can be formedusing the material given as a material for the insulating films 1341 aand 1341 b described in Embodiment Mode 1. The gate insulating film 1022is formed by a CVD method, a sputtering method, or the like.

Subsequently, a semiconductor film is formed over the gate insulatingfilm 1022. Here, as the semiconductor film, a microcrystallinesemiconductor film 1023 and an amorphous semiconductor film 1024 arestacked. In a reaction chamber of a plasma CVD apparatus, a depositiongas including silicon or germanium is mixed with hydrogen, and themicrocrystalline semiconductor film 1023 is formed by glow dischargeplasma. Hydrogen is diluted so that the flow rate of hydrogen is 10times to 2000 times, preferably 50 times to 200 times that of thedeposition gas including silicon or germanium, and the microcrystallinesemiconductor film is formed. A temperature for heating the substrate isfrom 100° C. to 300° C., preferably from 120° C. to 220° C.

In the film formation processing of the microcrystalline semiconductorfilm, helium may be added to a reaction gas, in addition to silane andhydrogen. Helium has the highest ionization energy among all the gases,which is 24.5 eV, and has a metastable state at about 20 eV, which is alittle lower than the ionization energy. Therefore, only about 4 eV,which corresponds to the difference therebetween, is necessary forionization while the discharging continues. Therefore, a dischargestarting voltage of helium shows the lowest value in all gases. By suchcharacteristics, plasma can be held stably with helium. In addition,uniform plasma can be formed so that plasma density is uniform even whenthe area of the substrate on which a microcrystalline semiconductor filmis deposited is increased.

As the amorphous semiconductor film 1024, an amorphous semiconductorfilm can be formed by a plasma CVD method using a deposition gasincluding silicon or germanium. Alternatively, by dilution of adeposition gas including silicon or germanium with one or plural kindsof noble gas elements selected from among helium, argon, krypton, andneon, an amorphous semiconductor film can be formed. Furtheralternatively, a hydrogen-containing amorphous semiconductor film can beformed using hydrogen with a flow rate of greater than or equal to 1time and less than or equal to 10 times, more preferably, greater thanor equal to 1 time and less than or equal to 5 times as high as that ofa silane gas. In addition, a halogen such as fluorine, chlorine,bromine, or iodine may be added to the above hydrogenated semiconductorfilm.

In the formation step of the microcrystalline semiconductor film 1023and the amorphous semiconductor film 1024, glow discharge plasma isgenerated by applying high-frequency power with a frequency of 1 MHz to20 MHz, typically 13.56 MHz or 27.12 MHz, or high-frequency power with afrequency of 20 MHz to about 120 MHz, typically 60 MHz.

Also, as the amorphous semiconductor film 1024, an amorphoussemiconductor film can be formed by sputtering with hydrogen or a noblegas, using a semiconductor such as silicon or germanium as a target.

Note that after formation of the microcrystalline semiconductor film1023, the amorphous semiconductor film 1024 is preferably deposited at atemperature of 300° C. to 400° C. by a plasma CVD method. By this filmformation processing, hydrogen is supplied to the microcrystallinesemiconductor film 1023, and the same effect as hydrogenizing themicrocrystalline semiconductor film 1023 can be obtained. That is, theamorphous semiconductor film 1024 is deposited over the microcrystallinesemiconductor film 1023, whereby hydrogen is dispersed in themicrocrystalline semiconductor film 1023, so that dangling bonds can beterminated.

Formation of the amorphous semiconductor film 1024 or the amorphoussemiconductor film including hydrogen, nitrogen, or halogen on a surfaceof the microcrystalline semiconductor film 1023 can prevent surfaces ofcrystal grains included in the microcrystalline semiconductor film 1023from being native oxidized. In particular, in a region where anamorphous semiconductor is in contact with microcrystalline grains, acrack is likely to be caused due to local stress. If this crack isexposed to oxygen, the crystal grains are oxidized to form siliconoxide. However, an amorphous semiconductor film is formed on the surfaceof the microcrystalline semiconductor film 1023, whereby oxidation ofmicrocrystalline grains can be prevented. In a display device includinga thin film transistor to which high voltage (e.g., about 15 V) isapplied, typically, in a liquid crystal display device, if the amorphoussemiconductor film is formed to have a large thickness, drain withstandvoltage is increased, so that deterioration of the thin film transistorcan be prevented even if high voltage is applied to the thin filmtransistor.

The energy gap of the amorphous semiconductor film 1024 is larger thanthat of the microcrystalline semiconductor film 1023; the resistivity ofthe amorphous semiconductor film 1024 is higher than that of themicrocrystalline semiconductor film 1023; and the mobility of theamorphous semiconductor film 1024 is as low as ⅕ to 1/10 of that of themicrocrystalline semiconductor film 1023. Therefore, in a thin filmtransistor to be formed later, the amorphous semiconductor film 1024formed between the microcrystalline semiconductor film 1023 and each ofsource and drain regions functions as a high-resistance region, whereasthe microcrystalline semiconductor film 1023 functions as a channelformation region. Accordingly, the off current of the thin filmtransistor can be reduced. When the thin film transistor is used as aswitching element of a display device, the contrast of the displaydevice can be improved.

Note that here, as the semiconductor film, a structure in which themicrocrystalline semiconductor film 1023 and the amorphous semiconductorfilm 1024 are stacked is used; however, a single layer of amicrocrystalline semiconductor film or an amorphous semiconductor filmmay be used. Further alternatively, a crystalline semiconductor film maybe formed with a single layer. A single layer of an amorphoussemiconductor film can be formed as a semiconductor film, whereby theprotection circuit illustrated in FIGS. 7A to 7C can be formed.

Next, a semiconductor film 1025 to which an impurity element impartingone conductivity type is added is formed over the semiconductor film(see FIG. 13B). The semiconductor film 1025 to which an impurity elementimparting one conductivity type is added is formed by a plasma CVDmethod using phosphine and a deposition gas including silicon orgermanium.

Then, after a resist is applied to the semiconductor film 1025 to whichan impurity element imparting one conductivity type is added, thesemiconductor film 1025 to which an impurity element imparting oneconductivity type is added, the amorphous semiconductor film 1024, andthe microcrystalline semiconductor film 1023 are etched into a desiredshape by using a resist mask formed by a photolithography step. Asillustrated in FIG. 13C, a microcrystalline semiconductor film 1031, anamorphous semiconductor film 1032, and a semiconductor film 1033 towhich an impurity element imparting one conductivity type is added areformed in the region where the thin film transistor is formed.

In the region where the protection circuit is formed, themicrocrystalline semiconductor films 1351 and 1352, the amorphoussemiconductor films 1353 and 1354, and the semiconductor films 1346 and1347 to which an impurity element imparting one conductivity type isadded are formed. Note that a stack including the microcrystallinesemiconductor film 1351, the amorphous semiconductor film 1353, and thesemiconductor film 1346 to which an impurity element imparting oneconductivity type is added, and a stack including the microcrystallinesemiconductor film 1352, the amorphous semiconductor film 1354, and thesemiconductor film 1347 to which an impurity element imparting oneconductivity type is added are separated from each other by theseparation portion 1348. Note that instead of separating the stacks ofthe semiconductor films, a depressed portion which is formed by partlyetching the stacks, may be provided. Then, the resist mask is removed.

Next, a conductive film 1034 is formed over the stacks of thesemiconductor films and the gate insulating film 1022. The conductivefilm 1034 can be formed using the material given as a material for thesource wiring and the second common lines 1312 a to 1312 c described inEmbodiment Mode 1. The conductive film 1034 is formed by a CVD method, asputtering method, a printing method, a droplet discharge method, or thelike.

Then, after a resist is applied to the conductive film 1034, theconductive film 1034 is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 13D, thesource wiring 1302 and a drain electrode 1042 are formed in the regionwhere the thin film transistor is formed. In addition, in the regionwhere the protection circuit is formed, the second common line 1312 isformed.

Next, the semiconductor film to which an impurity element imparting oneconductivity type is added is etched using a resist mask to form sourceand drain regions 1043 and 1044. Note that in the etching step, theamorphous semiconductor film 1032 is also partly etched. The amorphoussemiconductor film 1032 that is partly etched and has a depressedportion is referred to as an amorphous semiconductor film 1045. Thesource and drain regions and the depressed portion of the amorphoussemiconductor film can be formed in the same step. The depth of thedepressed portion in the amorphous semiconductor film 1045 is set to behalf to one third the thickness of the thickest region in the amorphoussemiconductor film 1045, so that the source and drain regions can have adistance therebetween. Therefore, leakage current between the source anddrain regions can be reduced. Then, the resist mask is removed.

Next, dry etching may be performed under such a condition that theamorphous semiconductor film which is exposed is not damaged and anetching rate with respect to the amorphous semiconductor film is low.Through this step, an etching residue on the amorphous semiconductorfilm 1045 between the source and drain regions, a residue of the resistmask, and a contamination source in an apparatus used for removal of theresist mask can be removed, whereby the source and drain regions can besurely insulated. As a result, leakage current of the thin filmtransistor can be reduced, so that a thin film transistor with small offcurrent and high withstand voltage can be manufactured. A gas containingchlorine, a gas containing fluorine, or the like can be used for anetching gas, for example.

Through the above steps, the thin film transistor is formed in the pixelportion. In the peripheral portion, the protection circuit can beformed.

Next, as illustrated in FIG. 13E, a protective insulating film 1051 isformed over the second common line 1312, the source wiring 1302, thedrain electrode 1042, and the gate insulating film 1022. The protectiveinsulating film 1051 can be formed using a silicon nitride film, asilicon nitride oxide film, a silicon oxide film, or a siliconoxynitride film. The protective insulating film 1051 prevents entry of acontaminating impurity such as an organic matter, a metal, or watervapor included in the air; thus, a dense film is preferably used for theprotective insulating film 1051.

Next, a planarization film 1052 may be formed over the protectiveinsulating film 1051. The planarization film 1052 can be formed using anorganic insulating film such as a film of an acrylic resin, polyimide,an epoxy resin, or siloxane polymer. The planarization film 1052 isformed using a photosensitive organic resin here. Then, after theplanarization film 1052 is exposed to light and developed, theprotective insulating film 1051 is exposed, as illustrated in FIG. 13F.Next, the protective insulating film 1051 is etched using theplanarization film 1052 as a mask, and a contact hole which exposes partof the drain electrode 1042 is formed.

Next, a pixel electrode 1061 is formed in the contact hole. Here, aftera conductive film is formed over the planarization film 1052, a resistis applied to the conductive film. Next, the conductive film is etchedusing a resist mask formed by a photolithography step with the use of aphotomask, and the pixel electrode 1061 is formed. Note that the rightside of FIG. 13F corresponds to the cross-sectional view taken alongline Q-R of FIG. 14.

The pixel electrode 1061 can be formed using a light-transmittingconductive material such as an indium oxide containing tungsten oxide,an indium zinc oxide containing tungsten oxide, an indium oxidecontaining titanium oxide, an indium tin oxide containing titaniumoxide, an indium tin oxide (hereinafter, also referred to as “ITO”), anindium zinc oxide, or an indium tin oxide to which silicon oxide isadded.

Alternatively, the pixel electrode 1061 can be formed using a conductivecomposition containing a conductive high-molecular compound (alsoreferred to as a conductive polymer). The pixel electrode formed usingthe conductive composition preferably has a sheet resistance which isequal to or less than 10000 Ω/square and a transmittance which isgreater than or equal to 70% at a wavelength of 550 nm. Further, theresistance of the conductive polymer included in the conductivecomposition is preferably equal to or less than 0.1 Ω·cm.

As the conductive polymer, a so-called π-electron conjugated conductivepolymer can be used. As examples thereof, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of more than two kinds of them, and thelike can be given.

Here, an ITO film is formed as the pixel electrode 1061 by a sputteringmethod, and then a resist is applied to the ITO film. Next, the resistis exposed to light and developed using a photomask, thereby forming aresist mask. Then, the ITO film is etched using the resist mask to formthe pixel electrode 1061. Then, the resist mask is removed.

As described above, the protection circuit illustrated in FIG. 8 can beformed at the same time as formation of the thin film transistor.Accordingly, through a small number of steps, an element substrate whichhas the protection circuit and the thin film transistor and can be usedfor a display device can be formed. In addition, an element substratewhose frame can be narrowed can be formed.

Next, a manufacturing process of the protection circuit illustrated inFIG. 9 and a thin film transistor is described with reference to FIGS.13A to 13F, FIGS. 15A to 15E, and FIG. 16. Note that a cross section ofa region where the protection circuit is formed is illustrated on theleft side of FIGS. 15A to 15E, and a cross section of a region where thethin film transistor is formed as a switching element in a pixel isillustrated on the right side of FIGS. 15A to 15E. In addition, theright side of FIGS. 15A to 15E is a cross-sectional view taken alongline Q-R illustrated in FIG. 16.

Through the step of FIG. 13A, the gate wiring 1301 is formed over thesubstrate 1001, as illustrated in FIG. 15A.

Next, the gate insulating film 1022 is formed over the gate wiring 1301and the substrate 1001.

Subsequently, a semiconductor film is formed over the gate insulatingfilm 1022. Here, as the semiconductor film, a microcrystallinesemiconductor film 1026 to which an impurity element that serves as adonor is added and an amorphous semiconductor film 1027 are stacked. Amethod of forming the microcrystalline semiconductor film 1026 to whichan impurity element that serves as a donor is added is describedhereinafter.

In a reaction chamber of a plasma CVD apparatus, a deposition gasincluding silicon or germanium, and hydrogen are mixed, and themicrocrystalline semiconductor film 1026 to which an impurity elementthat serves as a donor is added is formed by glow discharge plasma.Hydrogen is diluted so that the flow rate of hydrogen is 10 times to2000 times, preferably 50 times to 200 times that of the deposition gasincluding silicon or germanium, and the microcrystalline semiconductorfilm is formed. A temperature for heating the substrate is from 100° C.to 300° C., preferably from 120° C. to 220° C. In addition, themicrocrystalline semiconductor film 1026 to which an impurity elementthat serves as a donor is added can be formed by mixing a gas includingphosphorus, arsenic, antimony, or the like with the above source gas.Here, phosphine can be mixed with silane and hydrogen and/or a noblegas, and a microcrystalline silicon film including phosphorus can beformed by glow discharge plasma as the microcrystalline semiconductorfilm 1026 to which an impurity element that serves as a donor is added.

In the formation step of the microcrystalline semiconductor film 1026 towhich an impurity element that serves as a donor is added, glowdischarge plasma is generated by applying high-frequency power with afrequency of 1 MHz to 20 MHz, typically 13.56 MHz or 27.12 MHz, orhigh-frequency power with a frequency of 20 MHz to about 120 MHz,typically 60 MHz.

As a typical example of the deposition gas including silicon orgermanium, SiH₄, Si₂H₆, GeH₄, Ge₂H₆, and the like can be given.

Note that instead of forming the microcrystalline semiconductor film1026 to which an impurity element that serves as a donor is added, aninsulating film to which an impurity element that serves as a donor isadded may be formed as the gate insulating film 1022 and a semiconductorfilm that does not include an impurity element that serves as a donormay be formed thereover. For example, a silicon oxide film, a siliconnitride film, a silicon oxynitride film, or a silicon nitride oxide filmincluding an impurity element (phosphorus, arsenic, or antimony) thatserves as a donor can be formed as a gate insulating film. In addition,when the gate insulating film 1022 has a stacked structure, an impurityelement that serves as a donor may be added to a layer which is incontact with the microcrystalline semiconductor film or a layer which isin contact with the substrate 1001.

As a method of forming an insulating film to which an impurity elementthat serves as a donor is added as the gate insulating film 1022, a gateinsulating film may be formed using a gas including an impurity elementthat serves as a donor as well as a source gas of the gate insulatingfilm. For example, a silicon nitride film including phosphorus can beformed by a plasma CVD method using silane, ammonium, and phosphine. Inaddition, a silicon oxynitride film including phosphorus can be formedby a plasma CVD method using silane, dinitrogen monoxide, ammonium, andphosphine.

Before the gate insulating film 1022 is formed, a gas including animpurity element that serves as a donor may be supplied to a reactionchamber of a film formation apparatus, and the surface of the gateinsulating film 1022 and an inner wall of the reaction chamber mayadsorb the impurity element that serves as a donor. After that, the gateinsulating film 1022 is formed, and then a microcrystallinesemiconductor film is formed. Since the gate insulating film 1022 andthe microcrystalline semiconductor film are deposited while taking inthe impurity element that serves as a donor, the microcrystallinesemiconductor film 1026 to which an impurity element that serves as adonor is added can be formed.

Alternatively, before the microcrystalline semiconductor film 1026 towhich an impurity element that serves as a donor is added is formed, agas including an impurity element that serves as a donor may be suppliedto the reaction chamber of the film formation apparatus so that theimpurity element that serves as a donor may be adsorbed onto the gateinsulating film 1022 and the inner wall of the reaction chamber. Then,by depositing the microcrystalline semiconductor film, themicrocrystalline semiconductor film is deposited while taking in theimpurity element that serves as a donor; therefore, the microcrystallinesemiconductor film 1026 to which an impurity element that serves as adonor is added can be formed.

Note that here, the amorphous semiconductor film is formed over themicrocrystalline semiconductor film 1026 to which the impurity elementthat serves as a donor is added; however, a microcrystallinesemiconductor film to which an impurity element that serves as a donoris added may be formed with a single layer.

Then, after a resist is applied to the amorphous semiconductor film, theamorphous semiconductor film 1027 and the microcrystalline semiconductorfilm 1026 to which an impurity element that serves as a donor is addedare etched into a desired shape by using a resist mask formed by aphotolithography step. As illustrated in FIG. 15B, a microcrystallinesemiconductor film 1036 to which an impurity element that serves as adonor and an amorphous semiconductor film 1038 are formed in the regionwhere the thin film transistor is formed. In addition, amicrocrystalline semiconductor film 1035 to which an impurity elementthat serves as a donor is added and an amorphous semiconductor film 1037are formed in the region where the protection circuit is formed. Then,the resist mask is removed.

Next, the amorphous semiconductor film 1024 is formed over the stacks ofthe semiconductor films and the gate insulating film. Then, thesemiconductor film 1025 to which an impurity element imparting oneconductivity type is added is formed over the amorphous semiconductorfilm 1024.

Then, after a resist is applied to the semiconductor film 1025 to whichan impurity element imparting one conductivity type is added, thesemiconductor film 1025 to which an impurity element imparting oneconductivity type is added, the amorphous semiconductor film 1024, theamorphous semiconductor film 1037, and the microcrystallinesemiconductor film 1035 to which an impurity element that serves as adonor is added are etched into a desired shape by using a resist maskformed by a photolithography step. As illustrated in FIG. 15C, theamorphous semiconductor film 1032 and the semiconductor film 1033 towhich an impurity element imparting one conductivity type is added areformed in the region where the thin film transistor is formed. Note thatthe amorphous semiconductor film 1032 and the semiconductor film 1033 towhich an impurity element imparting one conductivity type is addedoverlap the microcrystalline semiconductor film 1036 to which animpurity element that serves as a donor is added and the amorphoussemiconductor film 1038, as illustrated in FIG. 15C. In addition, a sidesurface of the microcrystalline semiconductor film 1036 to which animpurity element that serves as a donor is added is covered with theamorphous semiconductor film 1032.

In the region where the protection circuit is formed, themicrocrystalline semiconductor films 1361 and 1362 to which an impurityelement that serves as a donor is added, the amorphous semiconductorfilms 1363 and 1364, the amorphous semiconductor films 1365 and 1366,and the semiconductor films 1367 and 1368 to which an impurity elementimparting one conductivity type is added are formed. Note that a stackincluding the microcrystalline semiconductor film 1361 to which animpurity element that serves as a donor is added, the amorphoussemiconductor film 1363, the amorphous semiconductor film 1365, and thesemiconductor film 1367 to which an impurity element imparting oneconductivity type is added, and a stack including the microcrystallinesemiconductor film 1362 to which an impurity element that serves as adonor is added, the amorphous semiconductor film 1364, the amorphoussemiconductor film 1366, and the semiconductor film 1368 to which animpurity element imparting one conductivity type is added are separatedfrom each other (the separation portion 1348). Note that instead ofseparating the stacks of the semiconductor films, a depressed portionwhich is formed by partly etching the stacks, may be provided. Then, theresist mask is removed.

Next, the conductive film 1034 is formed over the stacks of thesemiconductor films and the gate insulating film 1022.

Then, after a resist is applied to the conductive film 1034, theconductive film 1034 is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 15D, thesource wiring 1302 and the drain electrode 1042 are formed in the regionwhere the thin film transistor is formed. In addition, in the regionwhere the protection circuit is formed, the second common line 1312 isformed.

Next, the semiconductor film to which an impurity element imparting oneconductivity type is added is etched using a resist mask to form thesource and drain regions 1043 and 1044. In the etching step, part of theamorphous semiconductor film 1032 is also etched. The amorphoussemiconductor film 1032 that is partly etched and has a depressedportion is referred to as the amorphous semiconductor film 1045. Thesource and drain regions and the depressed portion of the amorphoussemiconductor film can be formed in the same step. Then, the resist maskis removed.

Next, dry etching may be performed under such a condition that theamorphous semiconductor film 1045 which is exposed is not damaged and anetching rate with respect to the amorphous semiconductor film 1045 islow.

Through the above steps, the thin film transistor is formed in the pixelportion. In the peripheral portion, the protection circuit can beformed.

Next, through the steps illustrated in FIGS. 13E and 13F, the protectiveinsulating film 1051, the planarization film 1052, and the pixelelectrode 1061 to be connected to the drain electrode are formed asillustrated in FIG. 15E. Note that the right side of FIG. 15Ecorresponds to the cross-sectional view taken along line Q-R in FIG. 16.

Through the above steps, the protection circuit illustrated in FIG. 9can be formed at the same time as formation of the thin film transistor.Accordingly, through a small number of steps, an element substrate whichhas the protection circuit and the thin film transistor and can be usedfor a display device can be formed. In addition, an element substratewhose frame can be narrowed can be formed.

Next, a manufacturing process of the protection circuit illustrated inFIGS. 10A to 10C and a thin film transistor is described with referenceto FIGS. 13A to 13F, FIGS. 17A to 17D, and FIGS. 18A to 18F. Note that across section of a region where the protection circuit is formed isillustrated on the left side of FIGS. 17A to 17D, and a cross section ofa region where the thin film transistor is formed as a switching elementin a pixel is illustrated on the right side of FIGS. 17A to 17D.

Through the step of FIG. 13A, the gate wiring 1301 is formed over thesubstrate 1001, as illustrated in FIG. 17A.

Next, the gate insulating film 1022 is formed over the gate wiring 1301and the substrate 1001.

Subsequently, a semiconductor film is formed over the gate insulatingfilm 1022. Here, an amorphous semiconductor film is formed as thesemiconductor film. Note that here, the amorphous semiconductor film isformed with a single layer as the semiconductor film; however, amicrocrystalline semiconductor film or a crystalline semiconductor filmmay be formed with a single layer as the semiconductor film.Alternatively, a microcrystalline semiconductor film and an amorphoussemiconductor film maybe stacked as illustrated in FIGS. 13A to 13F.

Next, a semiconductor film to which an impurity element imparting oneconductivity type is added is formed over the semiconductor film. Then,after a resist is applied to the semiconductor film to which an impurityelement imparting one conductivity type is added, the amorphoussemiconductor film and the semiconductor film to which an impurityelement imparting one conductivity type is added are etched into adesired shape by using a resist mask formed by a photolithography step.An amorphous semiconductor film 1039 and the semiconductor film 1033 towhich an impurity element imparting one conductivity type is added areformed in the region where the thin film transistor is formed. Inaddition, the amorphous semiconductor films 1344 and 1345 and thesemiconductor films 1346 and 1347 to which an impurity element impartingone conductivity type is added are formed in the region where theprotection circuit is formed. Note that the stack including theamorphous semiconductor film 1344 and the semiconductor film 1346 towhich an impurity element imparting one conductivity type is added, andthe stack including the amorphous semiconductor film 1345 and thesemiconductor film 1347 to which an impurity element imparting oneconductivity type is added are separated from each other (the separationportion 1348). Note that instead of separating the stacks of thesemiconductor films, a depressed portion which is formed by partlyetching the stacks, may be provided. Then, the resist mask is removed.

Next, an insulating film 1062 a is formed over the stacks of thesemiconductor films and the gate insulating film. Note that the stackincluding the amorphous semiconductor film 1344 and the semiconductorfilm 1346 to which an impurity element imparting one conductivity typeis added, and the stack including the amorphous semiconductor film 1345and the semiconductor film 1347 to which an impurity element impartingone conductivity type is added are separated from each other (theseparation portion 1348). In addition, the insulating film 1062 a coversthe separation portion 1348.

Then, after a resist is applied to the insulating film 1062 a, theinsulating film 1062 a is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 17B, aninsulating film 1062 b is formed to expose part of the semiconductorfilm 1033 to which an impurity element imparting one conductivity typeis added in the region where the thin film transistor is formed. Then,the resist mask is removed.

Next, the conductive film 1034 is formed over the insulating film 1062 band the exposed semiconductor film 1033 to which an impurity elementimparting one conductivity type is added.

Then, after a resist is applied to the conductive film 1034, theconductive film 1034 is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 17C, thesource wiring 1302 and the drain electrode 1042 are formed in the regionwhere the thin film transistor is formed. In addition, in the regionwhere the protection circuit is formed, the second common line 1312 isformed.

Next, the semiconductor film to which an impurity element imparting oneconductivity type is added is etched using a resist mask to form thesource and drain regions 1043 and 1044. In the etching step, part of theamorphous semiconductor film 1039 is also etched. The amorphoussemiconductor film 1039 that is partly etched and has a depressedportion is referred to as the amorphous semiconductor film 1045. Thesource and drain regions and the depressed portion of the amorphoussemiconductor film can be formed in the same step. Then, the resist maskis removed.

Through the above steps, the thin film transistor is formed in the pixelportion. In the peripheral portion, the protection circuit can beformed.

Next, through the steps illustrated in FIGS. 13E and 13F, the protectiveinsulating film 1051, the planarization film 1052, and the pixelelectrode 1061 to be connected to the drain electrode 1042 are formed asillustrated in FIG. 17D.

Through the above steps, the protection circuit illustrated in FIGS. 10Ato 10C can be formed at the same time as formation of the thin filmtransistor. Accordingly, through a small number of steps, an elementsubstrate which has the protection circuit and the thin film transistorand can be used for a display device can be formed. In addition, anelement substrate whose frame can be narrowed can be formed.

Next, a manufacturing process of the protection circuit illustrated inFIGS. 11A to 11C and a thin film transistor is described with referenceto FIGS. 13A to 13F, and FIGS. 18A to 18F. Note that a cross section ofa region where the protection circuit is formed is illustrated on theleft side of FIGS. 18A to 18F, and a cross section of a region where thethin film transistor is formed as a switching element in a pixel isillustrated on the right side of FIGS. 18A to 18F.

Through the step of FIG. 13A, the gate wiring 1301 is formed over thesubstrate 1001. Next, the gate insulating film 1022 is formed over thegate wiring 1301 and the substrate 1001.

Then, after a resist is applied to the gate insulating film 1022, partof the gate insulating film 1022 is etched by using a resist mask formedby a photolithography step. As illustrated in FIG. 18A, a gateinsulating film 1071 is formed. Here, in the region where the protectioncircuit is formed, the gate wiring 1301 is exposed. Then, the resistmask is removed.

Subsequently, a semiconductor film is formed over the gate insulatingfilm 1071. Here, an amorphous semiconductor film 1029 is formed as thesemiconductor film. Note that here, the amorphous semiconductor film isformed with a single layer as the semiconductor film; however, amicrocrystalline semiconductor film or a crystalline semiconductor filmmay be formed with a single layer as the semiconductor film. Asillustrated in FIGS. 13A to 13F, a microcrystalline semiconductor filmand an amorphous semiconductor film may be stacked.

Subsequently, the semiconductor film 1025 to which an impurity elementimparting one conductivity type is added is formed over thesemiconductor film (see FIG. 18B). Then, after a resist is applied tothe semiconductor film 1025 to which an impurity element imparting oneconductivity type is added, the amorphous semiconductor film 1029 andthe semiconductor film 1025 to which an impurity element imparting oneconductivity type is added are etched into a desired shape by using aresist mask formed by a photolithography step. The amorphoussemiconductor film 1039 and the semiconductor film 1033 to which animpurity element imparting one conductivity type is added are formed inthe region where the thin film transistor is formed. In addition, theamorphous semiconductor films 1344 and 1345 and the semiconductor films1346 and 1347 to which an impurity element imparting one conductivitytype is added are formed in the region where the protection circuit isformed. Note that the stack including the amorphous semiconductor film1344 and the semiconductor film 1346 to which an impurity elementimparting one conductivity type is added, and the stack including theamorphous semiconductor film 1345 and the semiconductor film 1347 towhich an impurity element imparting one conductivity type is added areseparated from each other (the separation portion 1348). Note thatinstead of separating the stacks of the semiconductor films, a depressedportion which is formed by partly etching the stacks, may be provided.Then, the resist mask is removed.

Next, the insulating film 1062 a is formed over the stacks of thesemiconductor films and the gate insulating film (see FIG. 18C). Notethat the stack including the amorphous semiconductor film 1344 and thesemiconductor film 1346 to which an impurity element imparting oneconductivity type is added, and the stack including the amorphoussemiconductor film 1345 and the semiconductor film 1347 to which animpurity element imparting one conductivity type is added are separatedfrom each other (the separation portion 1348). In addition, theinsulating film 1062 a covers the separation portion 1348.

Then, after a resist is applied to the insulating film 1062 a, theinsulating film 1062 a is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 18D, theinsulating film 1062 b is formed to expose part of the semiconductorfilm 1033 to which an impurity element imparting one conductivity typeis added in the region where the thin film transistor is formed. Then,the resist mask is removed.

Next, the conductive film 1034 is formed over the insulating film 1062 band the exposed semiconductor film 1033 to which an impurity elementimparting one conductivity type is added.

Then, after a resist is applied to the conductive film 1034, theconductive film 1034 is etched into a desired shape by using a resistmask formed by a photolithography step. As illustrated in FIG. 18E, thesource wiring 1302 and the drain electrode 1042 are formed in the regionwhere the thin film transistor is formed. In addition, in the regionwhere the protection circuit is formed, the second common line 1312 isformed.

Next, the semiconductor film to which an impurity element imparting oneconductivity type is added is etched using a resist mask to form thesource and drain regions 1043 and 1044. In the etching step, part of theamorphous semiconductor film 1039 is also etched. The amorphoussemiconductor film 1039 that is partly etched and has a depressedportion is referred to as the amorphous semiconductor film 1045. Thesource and drain regions and the depressed portion of the amorphoussemiconductor film can be formed in the same step. Then, the resist maskis removed.

Next, dry etching may be performed under such a condition that theamorphous semiconductor film 1045 which is exposed is not damaged and anetching rate with respect to the amorphous semiconductor film 1045 islow.

Through the above steps, the thin film transistor is formed in the pixelportion. In the peripheral portion, the protection circuit can beformed.

Next, through the steps illustrated in FIGS. 13E and 13F, the protectiveinsulating film 1051, the planarization film 1052, and the pixelelectrode 1061 to be connected to the drain electrode are formed asillustrated in FIG. 18F.

Through the above steps, the protection circuit illustrated in FIGS. 11Ato 11C can be formed at the same time as formation of the thin filmtransistor. Accordingly, through a small number of steps, an elementsubstrate which has the protection circuit and the thin film transistorand can be used for a display device can be formed. In addition, anelement substrate whose frame can be narrowed can be formed.

Next, another method of manufacturing a thin film transistor, which isdifferent from the above modes, is described with reference to FIGS. 13Ato 13F, FIGS. 19A to 19E, FIGS. 20A to 20D, and FIG. 21. Here, amanufacturing process of the thin film transistor using a multi-tonemask is described. Note that a cross section of a region where theprotection circuit is formed is illustrated on the left side of FIGS.19A to 19E, and a cross section of a region where the thin filmtransistor is formed as the switching element in the pixel isillustrated on the right side of FIGS. 19A to 19E. In addition, theright side of FIGS. 19A to 19E is a cross-sectional view taken alongline Q-R illustrated in FIG. 21.

Through the step of FIG. 13A, the gate wiring 1301 is formed over thesubstrate 1001. Next, the gate insulating film 1022 is formed over thegate wiring 1301 and the substrate 1001.

Next, through the step of FIG. 13B, the microcrystalline semiconductorfilm 1023, the amorphous semiconductor film 1024, and the semiconductorfilm 1025 to which an impurity element imparting one conductivity typeis added are sequentially stacked over the gate insulating film 1022.Note that instead of the microcrystalline semiconductor film 1023 andthe amorphous semiconductor film 1024, a microcrystalline semiconductorfilm, an amorphous semiconductor film, or a crystalline semiconductorfilm may be formed with a single layer.

Then, after a resist is applied to the semiconductor film to which animpurity element imparting one conductivity type is added, themicrocrystalline semiconductor film 1023, the amorphous semiconductorfilm 1024, and the semiconductor film 1025 to which an impurity elementimparting one conductivity type is added are etched into a desired shapeby using a resist mask formed by a photolithography step. Amicrocrystalline semiconductor film 1073, an amorphous semiconductorfilm 1074, and a semiconductor film 1075 to which an impurity elementimparting one conductivity type is added are formed in the region wherethe thin film transistor is formed. In addition, the microcrystallinesemiconductor films 1351 and 1352, the amorphous semiconductor films1353 and 1354, and the semiconductor films 1346 and 1347 to which animpurity element imparting one conductivity type is added are formed inthe region where the protection circuit is formed. Then, the resist maskis removed (see FIG. 19A).

Note that the stack including the microcrystalline semiconductor film1351, the amorphous semiconductor film 1353, and the semiconductor film1346 to which an impurity element imparting one conductivity type isadded, and the stack including the microcrystalline semiconductor film1352, the amorphous semiconductor film 1354, and the semiconductor film1347 to which an impurity element imparting one conductivity type isadded are separated from each other (the separation portion 1348) in theregion where the protection circuit is formed. Note that instead ofseparating the stacks of the semiconductor films, a depressed portionwhich is formed by partly etching the stacks, may be provided.

Next, as illustrated in FIG. 19B, the conductive film 1034 is formedover the gate insulating film 1022, the semiconductor films 1346, 1347,and 1075 to which an impurity element imparting one conductivity type isadded.

Then, a resist is applied to the conductive film 1034. As the resist, apositive resist or a negative resist can be used. Here, a positiveresist is used.

Next, a resist is irradiated with light using a multi-tone mask, andthen the resist is developed, whereby a resist mask 1081 is formed.

Now, light exposure using the multi-tone masks 159 a and 159 b aredescribed with reference to FIGS. 20A to 20D.

A multi-tone photomask can achieve three levels of light exposure toobtain an exposed portion, a half-exposed portion, and an unexposedportion; one-time exposure and development process allows a resist maskwith regions of plural thicknesses (typically, two kinds of thicknesses)to be formed. Therefore, the use of a multi-tone photomask allows thenumber of photomasks to be reduced.

Typical examples of a multi-tone mask include a gray-tone mask 159 aillustrated in FIG. 20A and a half-tone mask 159 b illustrated in FIG.20C.

As illustrated in FIG. 20A, the gray-tone mask 159 a includes alight-transmitting substrate 163 provided with a light-blocking portion164 and a diffraction grating 165. The light transmittance of thelight-blocking portion 164 is 0%. The diffraction grating 165 has alight-transmitting portion in a slit form, a dot form, a mesh form, orthe like with intervals which are equal to or less than the resolutionlimit for light used for the exposure; thus, the light transmittance canbe controlled. The diffraction grating 165 can be in a slit form, a dotform, or a mesh form with regular intervals; or in a slit form, a dotform, or a mesh form with irregular intervals.

For the light-transmitting substrate 163, a light-transmittingsubstrate, such as a quartz substrate, can be used. The light-blockingportion 164 and the diffraction grating 165 can be formed using alight-blocking material such as chromium or chromium oxide, whichabsorbs light.

When the gray-tone mask 159 a is irradiated with light for exposure, alight transmittance 166 of the light-blocking portion 164 is 0% and thatof a region where neither the light-blocking portion 164 nor thediffraction grating 165 is provided is 100%, as illustrated in FIG. 20B.The light transmittance of the diffraction grating 165 can be controlledin a range of 10% to 70%. The light transmittance of the diffractiongrating 165 can be controlled with an interval or a pitch of slits,dots, or meshes of the diffraction grating 165.

As illustrated in FIG. 20C, the half-tone mask 159 b includes thelight-transmitting substrate 163 provided with a semi-transmissiveportion 167 and a light-blocking portion 168. The semi-transmissiveportion 167 can be formed using MoSiN, MoSi, MoSiO, MoSiON, CrSi, or thelike. The light-blocking portion 168 can be formed using alight-blocking material such as chromium or chromium oxide, whichabsorbs light.

When the half-tone mask 159 b is irradiated with light for exposure, alight transmittance 169 of the light-blocking portion 168 is 0% and thatof a region where neither the light-blocking portion 168 nor thesemi-transmissive portion 167 is provided is 100%, as illustrated inFIG. 20D. The light transmittance of the semi-transmissive portion 167can be controlled in a range of 10% to 70%. The light transmittance ofthe semi-transmissive portion 167 can be controlled with the material ofthe semi-transmissive portion 167.

After the light exposure using the multi-tone mask is performed,development is carried out, whereby the resist mask 1081 having regionswith different thicknesses can be formed, as illustrated in FIG. 19B. Inthe region where the protection circuit is formed, the resist mask 1081for forming the second common line is formed.

Next, with the resist mask 1081, the microcrystalline semiconductor film1073, the amorphous semiconductor film 1074, the semiconductor film 1075to which an impurity element imparting one conductivity type is added,and the conductive film 1034 are etched to be separated. As a result, amicrocrystalline semiconductor film 1083, an amorphous semiconductorfilm 1084, a semiconductor film 1085 to which an impurity elementimparting one conductivity type is added, and a conductive film 1086 canbe formed as illustrated in FIG. 19C.

Next, the resist mask 1081 is ashed. As a result, the area and thethickness of the resist mask are reduced. At this time, the resist in aregion with a small thickness (a region overlapping part of the gatewiring 1301) is removed to form a separated resist mask 1091, asillustrated in FIG. 19C.

Next, the conductive film 1086 is etched to be separated using theresist mask 1091. As a result, the source wiring 1302 and a drainelectrode 1096 can be formed as illustrated in FIG. 19D. When theconductive film 1086 is etched by wet etching using the resist mask1091, the conductive film 1086 is isotropically etched. As a result, thesource wiring 1302 and the drain electrode 1096 which have a smallerarea than the resist mask 1091 can be formed. In addition, theconductive film is etched using the resist mask, and the second commonline 1312 is formed.

Next, the semiconductor film 1085 to which an impurity element impartingone conductivity type is added is etched using the resist mask 1091 toform a pair of a source region 1093 and a drain region 1094. Note thatpart of the amorphous semiconductor film 1084 is also etched in theetching step. The amorphous semiconductor film that is partly etched isreferred to as an amorphous semiconductor film 1092. Note that theamorphous semiconductor film 1092 is provided with a depressed portion.The source and drain regions and the depressed portion of the amorphoussemiconductor film 1092 can be formed in the same step. Here, theamorphous semiconductor film 1084 is partly etched with use of theresist mask 1091 having a smaller area than the resist mask 1081, sothat end portions of the amorphous semiconductor film 1092 are locatedoutside the source region 1093 and the drain region 1094. In addition,the end portions of the source wiring 1302 and the drain electrode 1096are not aligned with the end portions of the source region 1093 and thedrain region 1094, and the end portions of the source region 1093 andthe drain region 1094 are formed in the outside of the end portions ofthe source wiring 1302 and the drain electrode 1096. Then, the resistmask 1091 is removed.

Next, dry etching may be performed under such a condition that theamorphous semiconductor film 1092 which is exposed is not damaged and anetching rate with respect to the amorphous semiconductor film 1092 islow.

As illustrated in FIG. 19D, the end portions of the source wiring 1302and the drain electrode 1096 are not aligned with those of the sourceregion 1093 and the drain region 1094 respectively, whereby the endportions of the source wiring 1302 and the drain electrode 1096 can havea larger distance therebetween; thus, leakage current or short-circuitbetween wirings can be prevented. Accordingly, an inverted staggeredthin film transistor can be manufactured.

Through the above steps, the thin film transistor is formed in the pixelportion. In the peripheral portion, the protection circuit can beformed.

Next, through the steps illustrated in FIGS. 1 3E and 13F, theprotective insulating film 1051, the planarization film 1052, and thepixel electrode 1061 to be connected to the drain electrode are formedas illustrated in FIG. 19E.

Through the above steps, the protection circuit can be formed at thesame time as formation of the thin film transistor. Accordingly, througha small number of steps, an element substrate which has the protectioncircuit and the thin film transistor and can be used for a displaydevice can be formed. In addition, an element substrate whose frame canbe narrowed can be formed.

Note that in FIGS. 13A to 13F, FIG. 14, FIGS. 15A to 15E, FIG. 16, FIGS.17A to 17D, FIGS. 18A to 18F, and FIGS. 19A to 19E, when the gate wiring1301 is formed, a plurality of opening portions may be provided in aregion intersecting the second common line 1312. As illustrated in FIGS.12A to 12C, with such a structure, the gate wiring can have a projectionpart at the intersection of the gate wiring 1301 and the second commonline 1312, whereby a structure in which a dielectric breakdown easilyoccurs can be formed between the gate wiring 1301 and the second commonline 1312.

Note that in this embodiment mode, an inverted staggered thin filmtransistor is used as a thin film transistor. Instead of this, a topgate thin film transistor can be used. In that case, a gate wiring of athin film transistor as well as a first common line lined up along thegate wiring may be formed, and a source wiring as well as a secondcommon line lined up along the source wiring may be formed.

(Embodiment Mode 5)

Next, a structure of a display panel which is one mode of the displaydevice will be described.

FIG. 22A illustrates a mode of a display panel in which a signal linedriver circuit 6013 which is separately formed is connected to a pixelportion 6012 formed over a substrate 6011. An element substrate providedwith the pixel portion 6012, a protection circuit 6016, and a scan linedriver circuit 6014 is formed using the element substrate described inthe above embodiment mode. When the signal line driver circuit is formedusing a transistor in which higher field-effect mobility can be obtainedcompared with a thin film transistor in which an amorphous semiconductorfilm is used, an operation of the signal line driver circuit whichdemands higher driving frequency than that of the scan line drivercircuit can be stabilized. Note that the signal line driver circuit 6013may be formed using a transistor using a single crystal semiconductorfor a channel formation region, a thin film transistor using apolycrystalline semiconductor for a channel formation region, or atransistor using an SOI for a channel formation region. The transistorusing an SOI includes a transistor using a single crystal semiconductorlayer provided over a glass substrate. The pixel portion 6012, thesignal line driver circuit 6013, and the scan line driver circuit 6014are each supplied with potential of a power source, a variety ofsignals, and the like via an FPC 6015. Further, the protection circuitdescribed in the above embodiment mode may be provided between thesignal line driver circuit 6013 and the FPC 6015 or between the signalline driver circuit 6013 and the pixel portion 6012. The protectioncircuit 6016 may be the protection circuit described in the aboveembodiment mode as well as a protection circuit formed using one or moreelements selected from among a thin film transistor, a diode, aresistor, a capacitor, and the like.

Note that the signal driver circuit and the scan line driver circuit mayboth be formed over the same substrate as that of the pixel portion.

Also, when the driver circuit is separately formed, a substrate providedwith the driver circuit is not always required to be attached to asubstrate provided with the pixel portion, and may be attached to, forexample, the FPC. FIG. 22B illustrates a mode of a panel of a displaydevice in which a signal line driver circuit 6023 is separately formedand an element substrate in which a pixel portion 6022, a protectioncircuit 6026, and a scan line driver circuit 6024 are formed over asubstrate 6021 is connected to an FPC 6025. The pixel portion 6022, theprotection circuit 6026, and the scan line driver circuit 6024 areformed using the thin film transistor described in the above embodimentmode. The signal line driver circuit 6023 is connected to the pixelportion 6022 via the FPC 6025 and the protection circuit 6026. The pixelportion 6022, the signal line driver circuit 6023, and the scan linedriver circuit 6024 are each supplied with potential of a power source,a variety of signals, and the like via the FPC 6025. The protectioncircuit 6026 described in the above embodiment mode is provided betweenthe FPC 6025 and the pixel portion 6022. The protection circuit 6026 maybe the protection circuit described in the above embodiment mode as wellas a protection circuit formed using one or more elements selected fromamong a thin film transistor, a diode, a resistor, a capacitor, and thelike.

Furthermore, only part of the signal line driver circuit or only part ofthe scan line driver circuit may be formed over the same substrate asthe pixel portion with use of the thin film transistor described in theabove embodiment mode, and the rest may be formed separately andelectrically connected to the pixel portion. FIG. 22C illustrates a modeof a panel of a display device in which an analog switch 6033 a includedin a signal line driver circuit is formed over a substrate 6031, overwhich a pixel portion 6032 and a scan line driver circuit 6034 areformed, and a shift register 6033 b included in the signal line drivercircuit is separately formed over a different substrate and attached tothe substrate 6031. The pixel portion 6032, a protection circuit 6036,and the scan line driver circuit 6034 are formed using the thin filmtransistor described in the above embodiment mode. The shift register6033 b included in the signal line driver circuit is connected to thepixel portion 6032 via an FPC 6035 and the protection circuit 6036. Thepixel portion 6032, the signal line driver circuit, and the scan linedriver circuit 6034 are each supplied with potential of a power source,a variety of signals, and the like via the FPC 6035. The protectioncircuit 6036 described in the above embodiment mode is provided betweenthe shift register 6033 b and the analog switch 6033 a. The protectioncircuit 6036 may be the protection circuit described in the aboveembodiment mode as well as a protection circuit formed using one or moreelements selected from among a thin film transistor, a diode, aresistor, a capacitor, and the like.

As illustrated in FIGS. 22A to 22C, in display devices of thisembodiment mode, all or part of the driver circuit can be formed overthe same substrate as the pixel portion, using the thin film transistordescribed in the above embodiment mode.

Note that there are no particular limitations on a connection method ofa separately formed substrate, and a known method such as a COG method,a wire bonding method, or a TAB method can be used. Further, aconnection position is not limited to the positions illustrated in FIGS.22A to 22C as long as electrical connection is possible. Also, acontroller, a CPU, a memory, or the like may be formed separately andconnected.

Note that the signal line driver circuit used here includes a shiftregister and an analog switch. In addition to the shift register and theanalog switch, another circuit such as a buffer, a level shifter, or asource follower may be included. Also, the shift register and the analogswitch are not always required to be provided, and for example, adifferent circuit such as a decoder circuit by which selection of signalline is possible may be used instead of the shift register, and a latchor the like may be used instead of the analog switch.

(Embodiment Mode 6)

The element substrate obtained according to the above embodiment mode,the display device and the like using the element substrate can be usedfor a panel of an active matrix display device. That is, the aboveembodiment mode can be implemented in any of electronic devicesincorporating the element substrate or the display device using theelement substrate in a display portion.

Examples of such electronic devices include cameras such as a videocamera and a digital camera, a head-mounted display (a goggle-typedisplay), a car navigation system, a projector, a car stereo, a personalcomputer, and a portable information terminal (e.g., a mobile computer,a cellular phone, and an e-book reader). FIGS. 23A to 23D illustrateexamples of such electronic devices.

FIG. 23A illustrates a television device. The television device can becompleted by incorporating a display panel into a housing, asillustrated in FIG. 23A. A main screen 2003 is formed using the displaypanel, and other accessories such as a speaker portion 2009 and anoperation switch are provided. In such a manner, a television device canbe completed.

As illustrated in FIG. 23A, a display panel 2002 using a display elementis incorporated into a housing 2001. The television device can receivegeneral TV broadcast by a receiver 2005, and can be connected to a wiredor wireless communication network via a modem 2004 so that one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed. Thetelevision device can be operated by using a switch built in the housingor a remote control unit 2006. Also, a display portion 2007 fordisplaying output information may also be provided in the remote controlunit 2006.

Further, the television device may include a sub-screen 2008 formedusing a second display panel for displaying channels, sound volume, andthe like, in addition to the main screen 2003. In this structure, themain screen 2003 may be formed using a liquid crystal display panel, andthe sub-screen 2008 may be formed using a light-emitting display panel.In addition, the structure may be set so that the main screen 2003 isformed using a light-emitting display panel and the sub-screen 2008 isformed using a light-emitting display panel, and the sub-screen 2008 iscapable of being turned on or off.

FIG. 24 is a block diagram of a main structure of a television device. Adisplay panel 900 is provided with a pixel portion 921. A signal linedriver circuit 922 and a scan line driver circuit 923 may be mounted onthe display panel 900 by a COG method.

As for other external circuits, the television device includes a videosignal amplifier circuit 925 which amplifies a video signal amongsignals received by a tuner 924; a video signal processing circuit 926which converts a signal output from the video signal amplifier circuit925 into a color signal corresponding to each color of red, green, andblue; a control circuit 927 which converts the video signal into aninput specification of a driver IC; and the like on an input side of thevideo signal. The control circuit 927 outputs signals to each of thescan line side and the signal line side. When digital driving isperformed, a structure may be employed in which a signal dividingcircuit 928 is provided on the signal line side and an input digitalsignal is divided into m signals to be supplied.

Among the signals received by the tuner 924, an audio signal istransmitted to an audio signal amplifier circuit 929, and an outputthereof is supplied to a speaker 933 through an audio signal processingcircuit 930. A control circuit 931 receives control information onreceiving station (receiving frequency) and volume from an input portion932 and transmits a signal to the tuner 924 and the audio signalprocessing circuit 930.

This embodiment mode is certainly not limited to the television deviceand is also applicable to various usages such as a monitor of a personalcomputer and a display medium having a large area, for example, aninformation display board at a train station, an airport, or the like,or an advertisement display board on the street.

The element substrate and the display device including the elementsubstrate which are described in the above embodiment mode are appliedto the main screen 2003 and the sub-screen 2008, so that massproductivity of the television device in which image quality such ascontrast or the like is improved can be increased. In addition, the sizeof the television device can be reduced.

FIG. 23B illustrates an example of a cellular phone 2301. The cellularphone 2301 includes a display portion 2302, an operation portion 2303,and the like. The element substrate and the display device including theelement substrate which are described in the above embodiment mode areapplied to the display portion 2302, so that mass productivity of thecellular phone in which image quality such as contrast or the like isimproved can be increased. In addition, the size of the cellular phonecan be reduced.

A portable computer illustrated in FIG. 23C includes a main body 2401, adisplay portion 2402, and the like. The element substrate and thedisplay device including the element substrate which are described inthe above embodiment mode are applied to the display portion 2402, sothat mass productivity of the computer in which image quality such ascontrast or the like is improved can be increased. In addition, the sizeof the computer can be reduced.

FIG. 23D illustrates a desk lamp including a lighting portion 2501, ashade 2502, an adjustable arm 2503, a support 2504, a base 2505, and apower supply switch 2506. The desk lamp is formed using thelight-emitting device, which is described in the above embodiment mode,for the lighting portion 2501. Note that the term ‘lighting appliance’also encompasses ceiling lights, wall lights, and the like. Use of theelement substrate and the display device including the element substratewhich are described in the above embodiment mode can increase massproductivity and provide inexpensive desk lamps. In addition, the sizeof the lighting appliance can be reduced.

FIGS. 25A to 25C illustrate an example of a structure of a smartphone towhich the above embodiment mode is applied. FIG. 25A is a front view,FIG. 25B is a rear view, and FIG. 25C is a development view. Thesmartphone has two housings 1111 and 1112. The smartphone has both afunction of a cellular phone and a function of a portable informationterminal, and incorporates a computer provided to conduct a variety ofdata processing in addition to verbal communication (voice calls).

The housing 1111 includes a display portion 1101, a speaker 1102, amicrophone 1103, operation keys 1104, a pointing device 1105, a frontcamera lens 1106, a jack 1107 for an external connection terminal, anearphone terminal 1008, and the like, while the housing 1112 includes akeyboard 1201, an external memory slot 1202, a rear camera 1203, a light1204, and the like. In addition, an antenna is incorporated in thehousing 1111.

Further, in addition to the above structure, the smartphone mayincorporate a non-contact IC chip, a small size memory device, or thelike.

The housing 1111 and housing 1112 (FIG. 25A) which are put together tobe lapped with each other are developed by sliding as illustrated inFIG. 25C. In the display portion 1101, the display device described inthe above embodiment mode can be incorporated, and display direction canbe changed depending on a use mode. Because the front camera lens 1106is provided in the same plane as the display portion 1101, thesmartphone can be used as a videophone.

The speaker 1102 and the microphone 1103 can be used for videophone,recording, playback, and the like without being limited to verbalcommunication. With use of the operation keys 1104, operation ofincoming and outgoing calls, simple information input of electronicmails or the like, scrolling of a screen, cursor motion, and the likeare possible.

If much information is needed to be treated, such as documentation, useas a portable information terminal, and the like, the use of thekeyboard 1201 is convenient. When the housing 1111 and the housing 1112which are put together to be lapped with each other (FIG. 25A) aredeveloped by sliding as illustrated in FIG. 25C and the smartphone isused as a portable information terminal, smooth operation can beconducted by using the keyboard 1201 and the pointing device 1105. Thejack 1107 for an external connection terminal can be connected to an ACadaptor and various types of cables such as a USB cable, and chargingand data communication with a personal computer or the like arepossible. Moreover, a recording medium is inserted in the externalmemory slot 1202, so that the smartphone can handle storage and movementof a larger amount of data.

In the rear surface of the housing 1112 (FIG. 25B), the rear camera 1203and the light 1204 are provided, and a still image and a moving imagecan be taken by using the display portion 1101 as a viewfinder.

Further, the smartphone may have an infrared communication function, aUSB port, a function of receiving one segment television broadcast, anon-contact IC chip, an earphone jack, or the like, in addition to theabove-described functions and structures.

When the display device described in the above embodiment mode is used,mass productivity can be increased. In addition, the size of thesmartphone can be reduced.

This application is based on Japanese Patent Application serial no.2008-034724 filed with Japan Patent Office on Feb. 15, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a thin filmtransistor over a substrate, the thin film transistor including a gatewiring, a gate insulating film over the gate wiring, and a source wiringand a drain wiring over the gate wiring; a first wiring over thesubstrate, the first wiring comprising a same material as the gatewiring; an insulating film over the first wiring, the insulating filmcomprising a same material as the gate insulating film; and a secondwiring over the insulating film, the second wiring comprising a samematerial as the source wiring and the drain wiring, wherein theinsulating film includes a depressed portion which is overlapped withthe first wiring and the second wiring, wherein the first wiring is notin contact with a bottom of the depressed portion, wherein part of thesecond wiring is provided inside the depressed portion, and wherein thefirst wiring, the insulating film and the second wiring form aprotection circuit.
 2. The display device according to claim 1, furthercomprising a microcrystalline semiconductor film provided between thefirst wiring and the second wiring.
 3. The display device according toclaim 1, further comprising a pixel portion including the thin filmtransistor, and wherein one of the first wiring and the second wiring iselectrically connected to the thin film transistor.
 4. The displaydevice according to claim 1, further comprising a pixel portionincluding the thin film transistor and a terminal portion connected to asemiconductor integrated circuit or a flexible printed circuit, andwherein the protection circuit is provided between the pixel portion andthe terminal portion.
 5. The display device according to claim 1,further comprising a pixel portion including the thin film transistorand a terminal portion connected to a semiconductor integrated circuitor a flexible printed circuit, and wherein the protection circuit, thepixel portion and the terminal portion are provided over the substrate.6. The display device according to claim 1, wherein the display deviceis incorporated in one selected from the group consisting of atelevision device, a cellular phone, and a portable computer.
 7. Adisplay device comprising: a thin film transistor over a substrate, thethin film transistor including a gate wiring, a gate insulating filmover the gate wiring, and a source wiring and a drain wiring over thegate wiring; a first wiring over the substrate, the first wiringcomprising a same material as the gate wiring; an insulating film overthe first wiring, the insulating film comprising a same material as thegate insulating film; and a second wiring over the insulating film, thesecond wiring comprising a same material as the source wiring and thedrain wiring, wherein the insulating film includes a depressed portionwhich is overlapped with the first wiring and the second wiring, whereinthe first wiring is not in contact with a bottom of the depressedportion, wherein the first wiring includes an opening portion which isoverlapped with the depressed portion, wherein part of the second wiringis provided inside the depressed portion, and wherein the first wiring,the insulating film and the second wiring form a protection circuit. 8.The display device according to claim 7, further comprising amicrocrystalline semiconductor film provided between the first wiringand the second wiring.
 9. The display device according to claim 7,further comprising a pixel portion including the thin film transistor,and wherein one of the first wiring and the second wiring iselectrically connected to the thin film transistor.
 10. The displaydevice according to claim 7, further comprising a pixel portionincluding the thin film transistor and a terminal portion connected to asemiconductor integrated circuit or a flexible printed circuit, andwherein the protection circuit is provided between the pixel portion andthe terminal portion.
 11. The display device according to claim 7,further comprising a pixel portion including the thin film transistorand a terminal portion connected to a semiconductor integrated circuitor a flexible printed circuit, and wherein the protection circuit, thepixel portion and the terminal portion are provided over the substrate.12. The display device according to claim 7, wherein the display deviceis incorporated in one selected from the group consisting of atelevision device, a cellular phone, and a portable computer.